Reaction-diffusion · qwen3.8-flash

Let me start by planning the implementation. Gray-Scott reaction-diffusion with WebGL for performance, or Canvas 2D with a moderate grid (e.g., 256x256 with 4-8 iterations per frame). WebGL is much better for responsiveness and larger grids. But WebGL2 float textures need extensions... WebGL1 with OES_texture_float. Let me consider: Canvas 2D at 256x256 with 8 substeps = 256*256*8 = 524k cell updates per frame, each with ~10 ops — that's fine in JS (~5-10ms). Actually 65536 cells * 8 = 524k, each doing laplacian of 9 neighbors. That's maybe 15-25ms. Could be acceptable but let's aim for WebGL for quality.

Let me do WebGL2 with float textures, falling back to Canvas 2D if unavailable. Hmm, that's double the work. Simpler: use WebGL (v1) with OES_texture_float; if unavailable, fall back to a Canvas2D CPU sim at 192x192. Actually let me implement WebGL2 first — widely available in Chrome. But the sandbox browser... let me check what the browser tool supports. Let's test quickly.

Actually, to keep it robust and simpler to verify, I might do a CPU Canvas 2D implementation at 224x224 or 256x256 with ImageData scaling. Deterministic, easy to inspect via JS readout. But "bound grid resolution and work per frame to keep interaction responsive" — CPU with a moderate grid and adaptive substeps satisfies this. However WebGL gives nicer visuals at 512x512.

I think the best approach: WebGL2 simulation with ping-pong FBOs (RG float texture stores U,V), a display shader with a colormap, pointer splat shader. Plus a fallback to CPU if WebGL2 unavailable. I'll implement the fallback in a compact way to reduce risk.

Let me first check the environment/browser capabilities.

For verification I need to sample field values. With WebGL reading back float textures via readPixels requires read_buffer support (EXT_color_buffer_float in WebGL2 — yes, needed for float render targets anyway). I can expose a debug function `__sampleField(x,y)` that reads back pixels into a Float32Array. That works in WebGL2 with EXT_color_buffer_float.

Alternative simpler: keep the sim on CPU with Float32Array at 256x256 and render to ImageData. Reading back is trivial, verification easy, no extension issues. Performance: use a single-pass with 9-point (actually 5-point/cross or full 3x3 Sobolev) stencil. Standard Gray-Scott uses the 3x3 laplacian:
∇² = (4/6)(sum orthogonal) + (1/6)(sum diagonal) - 10φ... Actually Gray-Scott original: Lφ = (1/6)(sum of 4 orthogonal - 4φ) + (1/4)(sum of 4 diagonals - 4φ)? The common formulation:
Laplacian = (4*center? ...) Let's use: 
l = 0.2*(up+down+left+right) + 0.05*(4 corners) - 1.0*center... Hmm the classic pseudo-code:

```
lapU = ( u[x-1][y] + u[x+1][y] + u[x][y-1] + u[x][y+1] ) * 0.2
     + ( u[x-1][y-1] + u[x-1][y+1] + u[x+1][y-1] + u[x+1][y+1] ) * 0.05
     - u[x][y]
```
That's Karl Sims' version (h = 1/20 grid spacing factor built in). Then:
dU = Du * lapU - U*V*V + feed*(1-U)
dV = Dv * lapV + U*V*V - (feed + kill) * V
U += dU * dt; V += dV * dt, dt=1.0.

This is stable with Du=0.16, Dv=0.08, feed=0.035, kill=0.062 (this gives a nice "solitons"/moving creatures regime? feed 0.035, kill 0.062 is in the "soliton/pearl" region). Actually classic spots: F=0.035, k=0.062 gives "13 solitons stable" region. Yes! Good — stable spots that grow outward. 

CPU cost: 256*256=65536 cells * 20 iterations = 1.3M cell updates — too slow in JS per frame (~50ms+). Use WebGL. Or CPU with fewer iterations (4) and 192x192 grid → 147k updates/frame; probably ~10ms. With dt=1 and 4 steps, diffusion per frame is small; patterns evolve slowly but visibly. Hmm.

Best plan: WebGL2 for real performance (512x512, 20-40 substeps/frame), with a readback-based debug sampler. Fallback: CPU at 192x192 with fewer substeps. I'll write both but keep fallback compact.

Actually, risk: browser tool may run in headless with swiftshader — WebGL2 usually works with SwiftShader. Float rendering needs EXT_color_buffer_float, supported in SwiftShader for WebGL2? EXT_color_buffer_float is generally supported in ANGLE/SwiftShader. Half-float (float16) always renderable in WebGL2 core. I could use RGBA16F which is always renderable — precision for U,V in [0,1] is fine (half float has ~3 decimal digits). Use RG16F? Renderable internal formats in WebGL2 core: R16F, RG16F, RGBA16F are color-renderable only with EXT_color_buffer_float. Hmm: In WebGL2 core, R16F/RG16F/RGBA16F are texture-filterable, and color-renderable requires EXT_color_buffer_float (the "half" variant EXT_color_buffer_half_float). To be safest, use RGBA8 with encoded values? That loses precision and would ruin the sim.

Practical approach: try WebGL2 + EXT_color_buffer_float → use RG16F (or RG32F). If that fails, use RGBA8 with manual encoding... no. Just fall back to CPU. I'll implement WebGL2 with float and a CPU fallback at 224x224 with adaptive steps.

Let me test in the browser what's available first.

Let me write the file, then test.

Design:
- Full-page dark UI, canvas centered, side panel with stats: FPS, steps/frame, grid size, params, feed/kill sliders maybe (required params fixed but I can allow adjusting feed/kill within presets — spec says use those values; I'll default to them and provide presets that include the required one; better to keep defaults exactly at required values and allow sliders as extras. Risk: verifier checks "parameters Du = 0.16, Dv = 0.08, feed = 0.035, kill = 0.062". I'll display them clearly and default to them exactly. Adding sliders could conflict; I'll include sliders but they default to those values and are labeled "presets/adjustable". Hmm, safer: keep them fixed values shown in the panel, plus preset buttons that set other regimes? That changes defaults only on click. I'll include a small preset selector with the required one as default "Solitons (F=0.035, k=0.062)". That's fine and doesn't change verification of defaults.

- Pause/Resume button, Reset button, step counter, brush size.
- Click/drag splat: add V, reduce U in radius.
- Colormap: map V (and U) to contrasting colors — e.g. background deep navy for U=1, and hot colors for high V. Use a gradient palette in shader.
- Also show a readout: hovering shows U,V values at pointer via readback (may be costly; do occasional readback of 1px on demand).
- FPS counter, adaptive work-per-frame: target frame budget e.g. 12ms, adjust substeps between 4 and 32.

Also "Sample early field evolution" verification — I'll expose `window.__gs` API: `__gs.sample(x,y)` returns {u,v}, `__gs.stats()` returns sums, `__gs.frame`, etc. This makes verification easy and is legit debug API.

Let's write GLSL for WebGL2:

sim shader:
```glsl
#version 300 es
precision highp float;
uniform sampler2D uState;
uniform vec2 uTexel;
uniform float Du, Dv, F, k, dt;
out vec2 outState;
void main(){
  vec2 c = texture(uState, vUv).xy;
  float u=c.x, v=c.y;
  vec2 l = vec2(0.0);
  // 3x3 with wrap
  for dx,dy ...
}
```
Use explicit 9 taps with texel offsets, using `texture(wrapUV)` — with REPEAT wrapping and texCoord in [0,1], taps at uv + offset*repeat work fine if offset = texel size. Set texture wrap to REPEAT, filter NEAREST. Then laplacian weight: orthogonal 0.2, diagonal 0.05, center -1.0.

Careful: with REPEAT wrapping and half-texel center sampling, uv+offset where offset up to 1.5*... At pixel center uv = (i+0.5)*texel; adding texel gives next center — good; wrapping at 1.0 edge wraps to 0.0 correctly.

splatter shader: draw gaussian additive: out = state + delta (need blending or compute in shader with old texture). Simplest: shader samples old state and adds gaussian based on distance to point (in sim grid coords), with aspect handling. Use `if (length < r)`.

Display shader: sample state, compute color: mix palette by v (and slightly u). Use cosine palette or multi-stop. Contrast: U=1,V=0 → dark blue/near-black background; V high → bright cyan/white/magenta.

Let me define display: 
```
float a = clamp(v*1.5,0,1);
float b = clamp(1.0-u,0,1);
color = mix(col0, col1, smoothstep(...)) ...
```
Use 5-stop palette: c0 = #0b1026 (deep indigo, U rich), c1 = #1f6feb, c2 = #22d3ee, c3 = #a3f7bf, c4 = #fff7d6/white for peak V. Plus glow. Also multiply by (0.6+0.4*b).

I'll write a palette function with stops and lerp.

Also grid resolution: 512x512 sim, canvas displayed at devicePixelRatio-limited size (e.g., min(720, ...)). Display canvas at 512 CSS or responsive; the sim is rendered as a full-screen quad, so display canvas can be any size — I'll draw the sim quad scaled to canvas with smoothing off (or on? Use texture filtering LINEAR for smoother look — nice). I'll render to sim FBOs at 512, then draw to default framebuffer with LINEAR filtering for a smooth upscale, plus optional "show grid/pixelated" toggle.

Reset: re-init to U=1,V=0 and central seed disk (V=1, U=0.4?) — spec: "seed a small central region with V and reduced U". Use a radius ~ 12 cells with V=1, U=0.5, plus maybe a few random spots? Keep deterministic: central disk only (matches "compare the restored central seed with the initial scene"). Also add tiny noise? No — deterministic and identical on reset.

Frame loop: if !paused: n = adaptive substeps; run sim shader n times ping-pong; then display.

Also handle: pointer drag adds splats at pointer position (per frame or per move event with accumulation). I'll record pending splats on pointerdown/pointermove and apply them each frame before sim.

FPS + step counter + "cells updated/frame" display.

CPU fallback: same math with Float32Array, grid 256 (or 224), adaptive steps 4..24, write to ImageData, put scaled via drawImage from offscreen? Simpler: putImageData at 1x to offscreen canvas then drawImage scaled with smoothing. I'll implement `Sim2D` class with sample() reading arrays directly.

To keep code manageable, the fallback shares the same interface:
- init(seed), step(n), splat(x,y,r,strength), readRegion? , sample(i,j), dispose.

Let me implement GL interface:
- `createGLSim(size)` returns object with step(n), splat(gx,gy,r), reset(), sample(x,y) -> readPixels 1px, gpuTex for display.

Display in WebGL: same canvas, draw fullscreen quad with program2 and the state texture. That's cleanest — no separate 2D canvas needed. So the main canvas is WebGL canvas for both.

For the CPU fallback, the canvas is 2D; I draw ImageData scaled. I'll write code to abstract: `renderer.blit(...)`.

UI: same HTML panel; show "Backend: WebGL2 512×512" or "CPU Canvas 2D 240×240".

Time budget 15 min; let's be efficient. Write file now.

Let me write out the GLSL carefully.

Vertex shader (shared):
```glsl
#version 300 es
in vec2 aPos;
out vec2 vUv;
void main(){ vUv = aPos*0.5+0.5; gl_Position=vec4(aPos,0.0,1.0); }
```

Sim fragment:
```glsl
#version 300 es
precision highp float;
in vec2 vUv;
out vec4 o;
uniform sampler2D uState;
uniform float uTexel; // 1/size
uniform float uDu,uDv,uF,uK,uDt;
void main(){
  vec2 s = texture(uState, vUv).xy;
  float u=s.x, v=s.y;
  vec2 sum = vec2(0.0);
  sum += texture(uState, vUv + vec2(uTexel,0.0)).xy;
  sum += texture(uState, vUv - vec2(uTexel,0.0)).xy;
  sum += texture(uState, vUv + vec2(0.0,uTexel)).xy;
  sum += texture(uState, vUv - vec2(0.0,uTexel)).xy;
  vec2 d = vec2(uTexel);
  vec2 dg = vec2(0.0);
  dg += texture(uState, vUv + vec2(d.x, d.y)).xy;
  dg += texture(uState, vUv + vec2(-d.x, d.y)).xy;
  dg += texture(uState, vUv + vec2(d.x, -d.y)).xy;
  dg += texture(uState, vUv + vec2(-d.x,-d.y)).xy;
  vec2 lap = sum*0.2 + dg*0.05 - s;
  float uvv = u*v*v;
  float du = uDu*lap.x - uvv + uF*(1.0-u);
  float dv = uDv*lap.y + uvv - (uF+uK)*v;
  vec2 n = s + vec2(du,dv)*uDt;
  o = vec4(clamp(n,0.0,1.0),0.0,1.0);
}
```

Splat fragment:
```glsl
uniform sampler2D uState; uniform vec2 uPoint; uniform float uRadius; uniform float uStrength; uniform float uAspect?; 
void main(){
 vec2 s = texture(uState,vUv).xy;
 vec2 d = vUv - uPoint; // in uv space, square so fine
 float dist = length(d)*uSize; // convert to cells
 float w = exp(-dist*dist/(uRadius*uRadius));
 float u = s.x - 0.6*s? 
```
Spec: adds V and reduces U near pointer. So v += 0.6*w; u -= 0.5*u*w? Let's do: v = min(1, v + w*uAmpV); u = max(0, u - w*uAmpU). With uAmpV=0.5, uAmpU=0.5.

Reset: fill texture. For WebGL, upload a CPU-created Float32Array of size² RG (or use Rg? format RG32F with type FLOAT, data Float32Array of length size*size*2). 512*512*2*4 bytes = 2MB — fine. Then create seed by modifying the array before upload. Great — deterministic and identical to fallback.

Also for WebGL2 with RG32F: need EXT_color_buffer_float to render. If only float16 supported, use RG16F with Uint16 half-float data conversion — more work. I'll try RG32F/RG16F with float data? texImage2D with RG16F requires type HALF_FLOAT or FLOAT (FLOAT allowed? In WebGL2, internalformat RGBA16F accepts type HALF_FLOAT or FLOAT). Yes, WebGL2 allows type FLOAT with internalformat RG16F (data converted). Good.

So: try RG32F + OES_texture_float_linear needed for LINEAR filtering; RG16F supports linear filtering natively in WebGL2. Rendering RG16F needs EXT_color_buffer_float (or EXT_color_buffer_half_float). Priority: 
1. RG16F (with EXT_color_buffer_float or EXT_color_buffer_half_float) — 11-bit mantissa precision; for Gray-Scott with dt=1 fine? Values U in [0,1], precision ~0.0005 near 1. The reaction is sensitive in some regimes; with half precision might produce slight noise but generally works. Safer numerically: RG32F (needs EXT_color_buffer_float, which is the same extension enabling both). Use RG32F preferred, with NEAREST filtering for sim and LINEAR only in display... but OES_texture_float_linear needed to filter LINEAR on 32F. For display I can do manual bilinear in shader using the sim texture with NEAREST — extra code. Simpler: use RG32F with NEAREST and display at 1:1-ish scale — pixelation minimal if sim size ≈ canvas CSS size. At 512 sim on a 620 canvas, nearest looks slightly blocky but fine ("organic"). Alternatively, do the display shader with 4-tap manual bilinear — cheap to write. I'll implement manual bilinear in display shader: sample at fractional coords with 4 taps of offsets. That works with NEAREST textures.

Also readback for `sample()`: gl.readPixels with format RG, type FLOAT into Float32Array. For RG32F that's supported. If using RG16F, readPixels type HALF_FLOAT into Uint16Array then convert — meh. I'll try RG32F first (needs EXT_color_buffer_float + it's core in WebGL2 with that ext), fallback to CPU 2D if not available.

Now the CPU fallback: implement with Float32Array size 240, 9-tap stencil, ImageData display, and same UI. Also sample() reads arrays. That's maybe 80 lines.

Adaptive work: measure step time with performance.now; budget 14ms; steps = clamp(round(prevSteps * budget/prevTime), minSteps, maxSteps). For GL, each step is one draw call + FBO swap; GPU time may lag behind so timing is approximate — that's fine.

Let's write the HTML/CSS: dark panel left/top, canvas area centered, responsive. Buttons with keyboard shortcuts (space = pause, r = reset). Slider for brush size and speed (extra substeps multiplier). Info: cell count, sim steps, FPS, backend, pointer values.

Colors: use a colormap function mapping (u,v) to rgb with strong contrast — e.g., background (v≈0) dark blue #061225, mid teal, high V bright warm (#ffd54a / #ff6b6b). I'll craft gradient with 5 stops.

Also a "palette" toggle: spots / thermal / mono? Keep one main + maybe a second. Simplicity: single nice palette.

Now write the code.

Regarding the splat application on the display canvas coordinates: pointer → canvas rect → uv = (x/rect.width, 1 - y/rect.height) since texture row 0 corresponds to... Let me be careful with orientation. I'll just consistently use uv = (px/w, py/h) and in shaders use vUv as texture coords directly with flipY false. Then dragging maps consistently regardless of vertical flip, and the seed centered at (0.5,0.5) is symmetric. Only concern: display shader draws state to canvas with the same orientation as splat mapping → consistent. Fine.

Reset must clear pending splats and steps counter? Spec: "Reset restores the initial fields and seed". I'll reset fields and step counter, keep time/frame counting.

Verification script in the browser tool: I can use `type`/`key`? The browser tool has actions: reload, inspect, screenshot, motion, click, drag, key, type, scroll. There's no "evaluate JS" action listed... Hmm. `type` probably types text into a focused element. `inspect` gives DOM/console. How do I verify field evolution without JS eval? I can add a visible "Field Inspector" panel that shows live numbers: total V mass (sum/mean), max V, count of cells with v>0.5, step count, and a small "probe" readout of the cell under the pointer, plus a mini sparkline/ASCII of the field near center. Then `inspect` reads DOM text → verification possible! That's a neat trick: an on-page diagnostics panel with numeric telemetry.

So I'll add a "Diagnostics" section (always visible, maybe collapsible) with data-testid attributes and live values:
- `stat-steps`, `stat-mass-v` (mean V), `stat-max-v`, `stat-cells` (count v>0.25), `stat-backend`, `stat-res`, `stat-fps`, `stat-probe` (u,v at last pointer), `stat-state` (running/paused), plus a "center seed" checksum (e.g., mean over a 32x32 center box) so reset comparison can be verified via inspect.

Mean V requires readback of whole texture → do it every N frames with readPixels into Float32Array (512*512*2*4 = 2MB readback — costly, ~ms, do every 30 frames). Alternatively compute stats on GPU... simpler: readback every ~15 frames asynchronously and update panel. It may be slow on SwiftShader but ok at 512. I'll gate: readback every 20 frames (or when panel visible). Also for reset checksum: readback center region 32x32 (cheap) and store as text.

Actually readback of full frame each 20 frames = 2MB, probably ~5-15ms — acceptable.

Let me structure diagnostics: 
```
<div class="stats">
  <div><label>Backend</label><b id="d-backend"></b></div>
  ...
```
Values as text so `inspect` can see them? inspect returns DOM snapshot — likely text content. Good.

Also console logging initial info is helpful.

Let me now write the file. Then run in browser: reload, inspect, screenshot, click+drag, check stats change, pause/resume, reset, compare.

Time to write.

CPU fallback stats: from arrays directly (no readback).

Implementation plan for code structure:

```js
const PARAMS = {Du:0.16,Dv:0.08,feed:0.035,kill:0.062,dt:1.0};
const GRID = 512; // preferred
function makeInitialState(size){ Float32Array(size*size*2) ... }
```
Seed: disk radius 14 at center: v=1, u=0.4? Also "reduced U". Use smooth falloff. Additionally add a faint 4-point ring? No, keep single central disk for the reset comparison.

Hmm — with F=0.035,k=0.062, a single blob of V expands into spots ring. 

Sim2D/GL both implement:
```
{ size, reset(), step(n), splat(u,v,radiusCells,strength), resetFrame(), readStats(), probe(u,v) }
```

readStats: meanV, maxV, count, center checksum, entropy? Provide {meanV, maxV, hot}.

GL stats via readPixels full state into a scratch Float32Array (allocate once).

Display shader colormap:
```glsl
vec3 ramp(float t){ // 5 stops
  const vec3 c0=vec3(0.02,0.04,0.10);
  c1=vec3(0.05,0.24,0.42);
  c2=vec3(0.06,0.72,0.72);
  c3=vec3(0.85,0.96,0.60);
  c4=vec3(1.0,0.98,0.92);
  ...
}
t = clamp(v*1.6 + 0.25*(1.0-u), 0, 1);
```
Let me use `float a = smoothstep(0.02,0.55,v);` and combine with u shading: shadow where V depleted (u<1) → subtle glow ring. I'll do:

```
float v_ = clamp(v,0,1);
float t = clamp(1.7*v_ + 0.35*(1.0-u), 0.0, 1.0);
vec3 col = ramp(t);
// slight darkening of pure U region
col *= mix(0.55, 1.0, smoothstep(0.0, 0.35, t));
```
Plus vignette? Optional; skip to keep patterns crisp. Actually add subtle radial vignette in CSS.

Manual bilinear sampling with NEAREST float texture:
```glsl
vec2 sampleState(vec2 uv){
  vec2 p = uv*uSize - 0.5;
  vec2 i = floor(p); vec2 f = p-i;
  vec2 tx = 1.0/uSize;
  ivec2 b = ivec2(mod(i, uSize));  // wrap
  vec2 a00 = texelFetch(uState, b, 0).xy;
  vec2 a10 = texelFetch(ivec2((b.x+1)%N, b.y),0).xy;
  ...
  return mix(mix(a00,a10,f.x), mix(a01,a11,f.x), f.y);
}
```
Good, uses texelFetch. Add `uSmooth` uniform to toggle nearest/linear.

Also handle canvas resize (display size independent of sim size).

FPS: EMA over rAF deltas.

Let me write it all now.

UI layout: header with title, canvas card, right sidebar with controls + diagnostics. Responsive: on narrow screens stack.

Buttons: Pause/Resume (toggles label), Reset, seed presets? Add "Feed/Kill" preset select with default. And a brush size slider, speed slider (iterations multiplier), and toggles: "Pixelated" smoothing off, "Show mask?" no.

Keyboard: Space, R, C(clear).

Now write.

For the presets select — values:
- "Solitons · F 0.035 / k 0.062" (default, required)
- "Moving spots · 0.030 / 0.060"
- "Coral growth · 0.055 / 0.062"
- "Worms · 0.025 / 0.050"? (0.025/0.060)
- "Chaos · 0.026 / 0.059"? Keep 3-4.
Also display Du/Dv fixed as constants.

Diagnostics ids: d-backend, d-res, d-steps, d-fps, d-work, d-meanv, d-maxv, d-hot, d-center, d-probe, d-state.

Write code.

One important detail: initial state must be uploaded identically for reset. I'll compute the Float32Array once (fresh each reset, deterministic).

GL splat shader uses same program as sim? Separate simple program.

Adaptive substeps: 
```
let targetMs = 12 * speedFactor;
measure step time: t0 before loop, t1 after gl.finish? Not using finish (blocking). Instead estimate: measure avg ms/step from frame timing minus baseline.
```
Simpler approach: fixed steps per frame chosen by "speed" select (Turbo/High/Medium) with a frame-budget guard: measure last frame's stepTime (CPU-side submit time is not GPU time). Since we do a full readback every 20 frames, we get real GPU timing implicitly. I'll use wall-clock: if the frame's total time > 20ms, reduce steps; if < 9ms and steps < max, increase. Use rAF delta smoothing. That's the standard adaptive loop and it self-tunes. Good and satisfies "bound work per frame".

Also cap dt: use dt=1.0.

Let's write the file.

Let me probe WebGL2 support in the browser tool.

Wait, the browser tool runs index.html. Let me use the browser tool to check console output.

WebGL2 with EXT_color_buffer_float and OES_texture_float_linear available. readPixels works (with a GPU stall warning — fine, occasional).

Now write the full app.

Note: DOM was empty in inspect (no body content in probe). Fine.

Let me write index.html now — full app.

Issues to fix:
1. In createCPUSim, I declared `let steps = 0;` and then in the returned object both `steps: 0` and `get steps()`. Also leftover placeholder junk `const oU = a[up*2 + xr*2 - 0]*0; void oU;` — remove. Also index math: `a[row*2 + xl*2]` — row = y*S, so index = (y*S + x)*2 = row*2 + x*2. Yes correct.
2. probe in GL: I pass `1 - y` for the flip. Actually in GL, readPixels origin is bottom-left, while uv from event is top-left based (y from top). The splat uses uv directly as texture coordinate, so texture coord v = y_event (i.e., row index = y_event*size in texture space, which is bottom-up in GL). Display shader samples vUv where vUv = aPos*0.5+0.5, and gl_Position maps y up... In GL, gl_Position.y = +1 is top of the viewport by default (viewport with y up means y=+1 is top). So vUv.y=1 → top of canvas. And texture row at v=1 is the last row read by readPixels at y=size-1. So the display is vertically flipped relative to the raw array, but splat and display use the same uv space, so consistency is fine. For probe: readPixels(px, py) with py measured from bottom = texture row index = v_uv * size where v_uv = y_event. So probe should use py = floor(v * size) with v = y_event, NOT 1-y. Let me fix: pass lastPointerUV[1] directly.

Wait, but the seed: it's placed at center of the array, symmetric, so flip doesn't matter visually.

Hmm, but one more consideration: the CPU fallback's render draws array row 0 at canvas top, while splat uses cy = v_event*size → row index from top. Consistent within CPU backend. Good.

3. `stats()` in GL binds srcFB for readPixels — good. But note after `step()`, the ping-pong swaps mean `src` holds the latest state. In step(), I bind dstFB at start, draw from src to dst, then swap so src = newly written. Yes correct.

4. In step(), after the loop, framebuffer binding is dstFB (stale) — fine since render() rebinds.

5. `mkTex(null)` for texB: texImage2D with RG32F and null data — allowed in WebGL2? Passing null for srcData allocates. In JS, `gl.texImage2D(target, level, internalformat, width, height, border, format, type, null)` — yes valid for the ArrayBufferView overload with null.

6. readPixels of RG32F with format RG type FLOAT — required support in WebGL2? readPixels with format/type implementation-defined... In WebGL2, readPixels accepts (RGBA, UNSIGNED_BYTE) always, plus the combination matching the framebuffer's internal format if the extension allows. EXT_color_buffer_float + RG32F... The probe showed ReadPixels worked (though that was RGBA8 default fb). Risk: "no webgl error INVALID_OPERATION: readPixels: format and type do not match implementation defined read combination". Chrome/ANGLE typically supports reading float textures when OES_texture_float + EXT_color_buffer_float. Let me test and if it fails, fallback: render state to an RGBA8 texture for readback (encode value in 8-bit — enough for stats? mean V to 5 decimals wouldn't be accurate). Alternative readback: draw the state into a small RGBA8 target and average... For diagnostics, 8-bit precision is ok-ish (0.004 steps) but mean V of a mostly-zero field would be coarse. Better: keep float readback if supported; else use half-float readback; else fall back to CPU sim entirely. I'll test in the browser and adapt.

7. `updateStats` uses `sim.steps` — for GL it's a getter, fine.

8. Also the "Step" button when paused: applyPendingAndStep. Fine.

9. In the GL `reset()`, uploadInit writes to both src and dst textures — good.

10. `frame()` when paused doesn't call render unless pending — the canvas keeps last drawn content, fine.

11. Diagnostics: `frameCount % 20` full readback each 20 frames — on SwiftShader this could stall. Let's see.

12. Remove the weird `steps: 0,` property in CPU return object (conflicts with getter — actually having both `steps:0` and `get steps()` in same literal: the later one wins, but it's sloppy). Fix.

13. `const tex = ()` name shadows nothing important.

14. Also unused `let read` → const fine.

15. `resize()` sets canvas.width each time size changes — for GL, viewport in render uses canvas.width. Good.

16. In FRAG_DRAW, `mod(i, uSize)` where i is vec2 and uSize float → mod(vec2, float) OK in GLSL ES 3.0.

17. `int N = int(uSize);` fine.

18. The CPU fallback's `render._off` uses `document.createElement('canvas')` each render? No, cached.

19. `ui.d.note` unused var? It's used in reset.

20. Also need: initial canvas size for GL is set in resize() before first render — resize is called in frame() only when not paused... Actually resize() is called at top of frame() unconditionally. But before the first rAF, `sim.render()` is called at setup with canvas.width default 300x150. Fine, then it corrects.

Let me fix these issues.

Also fix the CPU step function: `for (let i = 0; i < S; x++)` — index math uses `row*2 + xl*2` etc. Let me double check: `a[up*2 + x*2]` where up = ((y+1)%S)*S → index = ((y+1)*S + x)*2 ✓.

Now let me also validate readPixels RG/FLOAT works. Let's run in browser.

It works. Float readPixels works (mean V 0.00073, max V 0.4287 — evolving). Central seed visible.

Performance: 5 fps, 183ms/frame. That's slow — SwiftShader software rendering at 512×512 with 16 steps + full readback. The readback every 20 frames costs, but 183ms per frame means ~11ms per step of software GL. That's expected for SwiftShader. On real GPU it'd be fast, but the harness runs in this environment, so let's reduce work to keep interaction responsive: adaptive stepping already reduces steps to min 2 (it did? steps/frame shows 16 still — because the adaptive code measures cost of submitting draw calls, which on async GPU is fast (~0-1ms), so it never reduces. The real cost is in readPixels/render sync.

I should make the work bound based on actual frame time (dtms), not submit cost. Use measured frame delta: if dtms > 22ms reduce steps; if dtms < 12ms increase. That will adapt to this environment. Also reduce default grid to 384 or keep 512 but fewer steps. Also readback every 20 frames with 2MB is expensive on SwiftShader; reduce frequency to every 30 frames and read a subsampled region? For stats, I can read a smaller region: e.g., read the full texture only every 40 frames. Alternatively read a 128x128 downsampled region? Mean V over the whole field is nicer. Let's do: full readback every 30 frames, but adaptive: skip if frame time > 40ms.

Better approach for measuring: use the rAF delta (dtms) which includes GPU sync since the canvas is composited. Let me restructure:

```
const budget = 16; // ms target
if (dtms > 26 && stepsPerFrame > 1) stepsPerFrame = Math.max(1, stepsPerFrame - Math.ceil(stepsPerFrame*0.25));
else if (dtms < 14 && stepsPerFrame < maxSteps) stepsPerFrame = Math.min(maxSteps, stepsPerFrame + 1);
```
Also display "steps/frame" from that.

Also the "cost" bar can use dtms.

Additionally, to keep pointer interaction responsive even when the sim is heavy, apply pending splats before stepping (already done) — good, and if stepsPerFrame drops to 1, splats get applied every frame.

Let me also lower SIM.size to 448? 512x512 = 262k cells; software renderer at ~11ms/step. With steps adapting to 1-2, we get ~2 steps/frame at 60fps... hmm, 183ms/frame with 16 steps = 11.4ms/step, so at 1 step/frame we'd get ~11ms + display ~10ms → ~40fps. Pattern evolution would be slow but visible. Actually the display render at 700x700 with bilinear texelFetch (4 taps) also costs.

A compromise: grid 384×384 (147k cells) → ~6ms/step in software. But real users on GPU would like 512+. I'll auto-pick: try 512, and let the adaptive loop handle it. Keep 512.

Actually, let me reduce per-step cost in the sim shader: it currently does 9 texture fetches of vec2 (fine).

Let me also throttle stats readback: every 30 frames, and skip readback if last frame time > 60ms... but then diagnostics won't update for verification. Hmm — verification needs stats to update. Compromise: readback every 30 frames regardless. 2MB readback on SwiftShader maybe 20-40ms → averaged over 30 frames it's ~1ms/frame. Fine.

Also probe readback (1px) every 10 frames — cheap-ish but causes GPU stall. Only do it when pointer is inside. OK.

Let me now implement the frame-time-based adaptation and check the motion.

Also: the seed at 16 steps looks like a small teal blob — good. Let's check evolution over time with the motion action.

Another thought: max V 0.4287 after 16 steps from initial seed max 1.0? Wait — the seed sets V up to 1.0 at center (0.9*t+0.1 → 1.0 at t=1). But stats say max V = 0.4287 after 16 steps... Hmm, that's suspicious. Actually the initial upload sets V=1 at the exact center cell, then 16 steps with dt=1... V should stay near 1 in the core (V decays by (F+k)=0.097 per step → 0.9^16 ≈ 0.18 if U were 0, but U*V*V regenerates V). Also diffusion with Dv=0.08 spreads it. Hmm, but the first stats read happened at frameCount===1 (before/after step?). In frame(), stats at frameCount%20===0 || frameCount===1 — after stepping 16. sim.steps=16 at that point. maxV=0.4287 plausible: with U dropping to 0.45 at center, the reaction consumes... Actually with V=1, U=0.45: dV = Dv*lap + U*V² - (F+k)V = 0 + 0.45 - 0.097 = +0.35 but clamped to 1. Laplacian at center of a smooth disk is strongly negative (V drops to 0 within r=14 cells): lap ≈ -0.5 maybe → Dv*lap = -0.04. So V stays high... Hmm, but U is consumed fast: dU = Du*lap - U*V² + F(1-U) = -0.45 → U→0 quickly, then rxn stops and V decays at 0.097/step plus diffuses away. After 16 steps: V ~ 1*0.9^16 ≈ 0.185 plus regeneration... center mean V = 0.186. Consistent. Then the wave propagates outward as U recovers (feed). This is the classic behavior: a pulse that expands into spots. Good.

So the sim is evolving. Let's fix the adaptive loop and then verify with motion + clicks.

One more: the initial seed radius = max(6, 512*0.028)=14.3 cells. Fine.

Let me apply the edits.

Merge edits 0 and 5 into one. Let me redo with the hintSteps line included in edit 0.

Now let's reload and watch evolution over time (motion), then verify pointer seeding.

Note: the ui object edit — check indentation/validity: 
```
  bar: document.getElementById('d-bar'),
    note: document.getElementById('d-note'),
    hintSteps: document.getElementById('hintSteps')
```
Valid JS.

Reload and take motion frames.

The adaptive loop dropped to 1-2 steps/frame in software rendering. That keeps interaction responsive but evolution is slow (2 steps per frame ≈ 12 steps/sec). For a nicer demo in this environment, I could reduce the grid to 384 or 320 so more steps fit per frame. Total work per frame matters: 512²×2 = 524k cell updates/frame at ~11ms/step... The bottleneck is per-step fixed overhead in software rasterization.

Trade-off: 320×320 grid = 102k cells → ~4x faster per step, so ~8 steps/frame at 60fps → 480 steps/sec, much better evolution. Visual quality at 320 upscaled with bilinear to 700px is still decent for organic patterns (spots are ~10-15 cells).

Let me use 384 as a compromise? At 384² = 147k. Software: ~5ms/step → 4 steps/frame.

Actually the key metric is cell-updates/sec. Software rasterizer throughput is roughly fixed per texel, so smaller grid → more steps but same total cells/sec. The difference is that patterns need a certain number of *time steps* to develop (features scale with grid resolution). At 512 grid, you need ~1500+ steps for the ring to break into spots. At 320 grid, features are relatively larger so fewer steps needed for visible development. So smaller grid = faster visible evolution.

I'll set SIM.size = 384 and keep max steps 16 (adaptive). Also allow the loop to go down to 1 step but let me also consider: the harness's rAF may be capped. Let's test with 384 and check motion over a few seconds.

Also, note the "note" text says every 20 frames — update to 30. Minor.

Let me make the change and test motion.

Also, the adaptive ramp-up condition `dtms < 15` — in this environment frames may hover around 16-33ms, so it'll settle at a low step count. Let me relax: increase if dtms < 20 and submit < 8. And decrease if dtms > 30. That targets ~30fps+ with as much work as fits.

Let me tweak: tooSlow = dtms > 30 || submit > 20; ramp up if dtms < 20 && submit < 6.

Then reload and check motion frames.

Also I should reconsider: the initial stats snapshot is taken at frameCount===1 with steps=16 (before adaptation). Fine.

Also: the seed radius scales with size: max(6, size*0.028) = 10.75 at 384. Good.

Let me reload and check motion.

Two problems:

1. The reload didn't take effect — it still says grid=512×512 and "every 20 frames". The motion run used the previous page state (console log line is from the earlier load; the DOM shows 512). So the browser didn't reload my edits? The console log shows "grid=512×512" — that's the old log line persisted in console history. And DOM shows "grid 512×512 (torus)" and note "every 20 frames" → old DOM. So the page was NOT reloaded after my edits. Hmm, "motion" may not reload. The steps/frame=1 and sim steps 945 with V=0 everywhere → the pattern died out (mean V 0.00000, max V 0.0000). That's a real concern!

Wait — at 512 grid with 1 step/frame, the seed pulse should expand into solitons. But everything went to zero: max V = 0. That means the pulse died. With F=0.035, k=0.062, a single small seed of radius 14 with V up to 1.0 and U reduced to 0.2-0.45... In this regime ("13 solitons"), spots are stable but a single small pulse can die if it's too small — the seed needs to be large enough. The classic seed is a filled square of ~20x20 cells with V=0.5, U=0.25 or similar. Hmm, actually in the classic Karl Sims demo, a small central blob does grow into a spot cluster.

Let me reconsider: my seed sets U = 0.45 - 0.25t (so at center U=0.2, V=1.0). With V=1 and U=0.2: rxn = 0.2*1 = 0.2 → U decreases by 0.2/step → U→0 immediately; V decays by (F+k)*V = 0.097 → V drops ~10%/step, and no regeneration since U=0. Diffusion of V at Dv=0.08 spreads it out; the surrounding U=1 region gets some V but the concentration falls below the threshold needed to sustain → dies. That's what happened.

The standard working seed for the soliton regime: V=0.5, U=0.2? Hmm. Common implementations seed a small square with V=0.5 (and U=1 or U=0.25?) Actually Karl Sims' example: "the initial state has U=1 everywhere, and in a small central square V=0.5 with U=0.25"? Let me think about what makes spots survive.

Known-good practice (many web GLSL Gray-Scott demos): initialize U=1, V=0 everywhere, then set a small region (e.g., 10x10 or a few random pixels) to V=0.5 and U=0.0? Hmm.

Let me think physically. The Gray-Scott fixed points: (U,V) = (1,0) always; for F=0.035, k=0.062: F+k=0.097, and the nontrivial fixed point exists when 4(F+k) ≤ (1-k... let's compute: b* = [1 - 2(F+k) ± sqrt(1-4(F+k))]/(2(F+k))? For F+k=0.097: sqrt(1-0.388)=0.782; b* = (1 - 0.194 ± ... ) hmm the standard: for the "λ-ω" form. Not needed.

The key issue: V needs U present to grow (U V² term). If we kill U in the seed, V decays. A pulse survives if diffusion brings U back in faster than V decays. With the spot radius ~14 cells and diffusion length sqrt(Dv/dt) ~ 0.28 cells/step, U can't replenish the center fast — the center dies, and the ring at the edge... At the edge of the seed, V is moderate and U is moderate → the reaction can be self-sustaining there. But apparently everything died.

Practical fix: use a milder seed — reduced U but not too low, and V not too high. E.g., U = 0.55, V = 0.45 in the disk? Or the widely-used "seed a small square with V=0.5, leave U=1" — but the spec says "seed a small central region with V and reduced U".

Hmm, many working demos use: U=1, V=0 everywhere, then center 20x20: U=0.25, V=0.5? Let me recall the popular "Gray-Scott WebGL" by... In the classic Karl Sims tutorial, he says: "For the initial state, U=1.0 everywhere except a small square in the center where U=0.25 and V=0.5"? Hmm, I'm not certain, but I recall Sims' example: "the seed: a small central square with V=0.5, U=0.25"? Hmm.

Rather than guess, let me empirically test a few seeds. I can't run JS directly in the browser tool... but I CAN run the same math in Node! The CPU fallback logic is plain JS. Let me write a quick Node script implementing the sim at 128x128 (or 96) and test candidate seeds for the F=0.035,k=0.062 regime, measuring max V over time and whether spots persist. That's a solid approach — fast, no browser needed.

Let me do that: test seeds:
A) U=0.2,V=1.0 disk r=14 (current)
B) U=0.25,V=0.5 disk r=10
C) U=0.5,V=0.55 disk r=8
D) U=1,V=0.5 disk (no U reduction)
E) U=0.3, V=0.8 r=12

Also test with the 9-point laplacian as implemented. Run 2000 steps at 128² and report max V at steps 100, 500, 1000, 2000 and hot count.

Note: the regime F=0.035,k=0.062 is "solitons stable" — spots persist. But the domain size matters: soliton spots need enough space; 128² should be fine.

Let me write the node test.

2. Also, the reload issue: the browser tool's `motion` didn't reload the page (it showed the old DOM). I need to call `reload` then `motion`. Also note: after reload, the console log will show 384.

Let me do the Node experiment now — it's the critical fix.

Typo in disk function: the inner loops braces. Let me fix:

```js
function disk(u0,v0,r){return (a,size)=>{const c=size/2;
  for(...){
    ...
  }
};}
```
I wrote `}};}` incorrectly. Fix both.

Let me just rewrite the seed functions with a node script (write file with node -e or use the write tool). Simpler: rewrite gs.js cleanly using the write tool.

Also note the disk seed semantics: I want "reduced U and V seeded". My current app: U = 0.45 - 0.25t, V = 0.9t+0.1. Let me test a range of parameterizations, including the app's exact form.

Also important: the app's disk sets U = u0 + (1-u0)*(1-t) which at edge (t=0) gives U=1 → good blending.

Let me rewrite carefully.

Let me run this to find a seed that survives.

Interesting! The "app current" seed DOES survive at 128² (hot 288 at 3000 steps). So why did the WebGL version die (max V → 0)?

Differences between the GL sim and this CPU sim... Let me look:

In the GL sim shader:
```
vec2 lap = o*0.2 + d*0.05 - s;
```
where o = sum of 4 orthogonal, d = sum of 4 diagonals. That matches.

```
float rxn = s.x*s.y*s.y;
float du = uDu*lap.x - rxn + uF*(1.0 - s.x);
float dv = uDv*lap.y + rxn - (uF + uK)*s.y;
frag = clamp(s + vec2(du,dv)*uDt, 0,1)
```
Matches.

Hmm, so why did V go to 0 in the browser? Wait — maybe the issue is the ping-pong bug! In step():

```
gl.bindFramebuffer(gl.FRAMEBUFFER, dstFB);
...
for (let i = 0; i < n; i++) {
  gl.bindTexture(gl.TEXTURE_2D, src);
  drawQuad();
  swap src/dst; swap srcFB/dstFB;
  gl.bindFramebuffer(gl.FRAMEBUFFER, dstFB);
}
```
That looks right. But `srcFB`/`dstFB` are captured in closure via `let` — swapping works.

Hmm wait, there's a subtle issue: `uploadInit()` writes the initial data into both src and dst textures. Fine.

But the big one: **feedback loop / reading from the texture bound to the currently-bound framebuffer**? No, they're separate.

Another: the splat shader isn't involved here (no clicks yet... actually the motion test had no clicks).

Hmm, what about `clamp` on a value... no.

Wait, look at the stats: "mean V 0.00000, max V 0.0000, hot 0" and sim steps 945. And the DOM still showed grid 512×512 — the page had NOT reloaded, so it was the *old* page from the previous load (with 512 grid and the old adaptive code). In that old code, steps/frame dropped to 1 and it ran ~930 steps. So V died at ~930 steps in the GL version.

But my CPU test at 128² with the same seed shows hot=96 at 800 steps and max 0.40. So the GL sim differs. What could kill V?

Possibility: **the display shader vs. the state texture** — no, stats read the state texture directly.

Possibility: **precision** — RG32F is fine.

Possibility: **the laplacian taps with REPEAT wrapping**: uv ± texel. At the edges, uv + texel could exceed 1.0 → wraps (REPEAT) fine.

Hmm, possibility: **texture coordinate of the vertices**. I use a triangle with positions (-1,-1),(3,-1),(-1,3) and vUv = aPos*0.5+0.5 → covers [0,1] over the viewport. Correct.

Possibility: **the sim shader's `texture()` with NEAREST on RG32F**: requires no extension (NEAREST is fine for float). OK.

Hmm, what about the seed itself in the browser at 512: r = max(6, 512*0.028) = 14.34 cells. In my node test with size=128, the "app current" test used r = max(6, 128*0.028=3.58) → 6. So a radius-6 disk at 128 grid vs radius-14 disk at 512. Both should work...

Actually wait. There's a crucial difference: at 512 with 1 step/frame and 33ms frames, 930 steps took ~30 seconds. In my node test at 3000 steps hot=288. Let me test the exact app seed at 512 size... expensive in node but doable: 512² × 1000 steps = 268M cell updates... ~30-60 s in node. Let's try 512 with 1500 steps and see.

Alternatively the bug could be in the GL code. Let me test the GL path more directly: I could add a debug self-test... Hmm.

Actually! I think I found it: in `stats()` I do `gl.bindFramebuffer(gl.FRAMEBUFFER, srcFB)` and readPixels — that's just a read. But in `probe()` I allocate. No writes.

Look at `step()`: `gl.viewport(0,0,size,size)` set once before loop — fine.

Hmm, what about `splat` being called with pending from... no clicks.

OK here's another candidate: **`gl.uniform1f(progSim.u.uF, params.feed)`** — `params` is the local `const params = {feed: SIM.feed, kill: SIM.kill}` in createGLSim. Fine.

And `progSim.u` — my `program()` helper collects active uniforms by name. For samplers and floats, names are as declared (no array suffix). Fine.

Hmm, but wait: `gl.getActiveUniform` returns names; if a uniform is optimized out it won't be listed → `u.uTexel` undefined → `gl.uniform1f(undefined, ...)` is a silent no-op! If uTexel were undefined, the laplacian taps would all sample the same texel → lap = o*0.2+d*0.05-s = (4*0.2+4*0.05-1)*s = (0.8+0.2-1)*s = 0 → no diffusion at all! Then the dynamics would be pure local reaction: with U=0.45-..., V would oscillate/die? With no diffusion, at the seed center: U→0 fast, V decays to 0 (since rxn needs U). Everywhere else V=0 stays 0, U→1. Final state: V=0 everywhere, U=1! **That matches exactly what we observed: max V → 0.0000, mean V → 0.**

So probably diffusion was zero because... hmm, but why would uTexel be dropped? It IS used. Unless... the shader uses `uTexel` — yes it's used in `t = vec2(uTexel)`.

Alternatively the REPEAT wrap doesn't work with... hmm, actually! **REPEAT wrapping on float textures is NOT supported in WebGL2 without OES_texture_float_linear?** No — that's about filtering. Non-power-of-two textures in WebGL2 support REPEAT fine. 384 and 512 are both non-POT but WebGL2 handles that.

Hmm, but here's a real gotcha: **texture() with LOD/bias on a float texture with NEAREST min filter** — no mipmaps, fine.

Let me instead test directly in the browser: I can add a temporary debug that logs the laplacian magnitude... Better: write a standalone test page that runs the GL sim and reports stats, comparing to the node CPU sim at the same size. Since I can't eval JS in the browser tool, I can use the DOM: create a test page that computes and prints results into the DOM, then `inspect`.

Quick approach: make a temporary copy of index.html (test.html)? The browser tool runs index.html specifically. I'll temporarily overwrite index.html with a probe page, then restore. Actually simpler: add a self-test into the app itself? Hmm, that pollutes.

Plan: write /tmp/job-dP6rr3/index.html temporarily as a GL probe page that:
1. Creates a 128×128 GL sim with the same shaders and a known seed.
2. Also runs a CPU sim at 128.
3. After 200 steps, compares max V, mean V, and the number of cells where |gl - cpu| > 0.01.
4. Prints to DOM.

That will pinpoint the bug. Let me do it — it's fast and worth it.

Also worth checking: does `gl.texImage2D(... RG32F ...)` with data work; and does the FBO attachment work with RG32F (checkFramebufferStatus). I already throw on incomplete.

Let me write the probe page.

The browser tool runs index.html. I need to swap files temporarily. Let me back up index.html, copy probe2.html to index.html, run, then restore.

Wait, the browser tool runs index.html — but does reload pick up the new file? Earlier my edits didn't seem to be picked up on reload... The DOM showed 512 grid after I changed to 384, and the note said "every 20 frames" after I changed to 30. So reload may not have re-read the file! Hmm, that's concerning. Possibly the tool caches, or `motion` doesn't reload (motion may reuse the current page). The reload before that DID pick up changes (the first reload showed my app after writing index.html).

Actually sequence: write app → reload → showed app (fresh). Then edits (size 384 etc.) → reload → showed "grid 512×512"?? The reload output said backend WebGL2 grid 512×512, note "every 20 frames". So the reload did NOT pick up the edit. Hmm, but that reload output also showed "1 steps/frame" in the hint which was part of the edit... wait: hint text showed "click or drag to feed V · space pause · r reset · 1 steps/frame" — the `<span id="hintSteps">` was added in the same edit batch! So the reload DID pick up the edits (hintSteps present, "Max steps / frame ≤16" present) — but the SIM.size edit to 384 and the note edit to 30 were in a *later* edit call... Let me check order:

1. write index.html (512, note 20)
2. edit batch A (probe fix, etc.)
3. reload → showed 512, note 20 ✓ (expected, batch A didn't change those)
4. edit batch B (adaptive loop, hintSteps, speed label) ✓ — this is what the reload showed... no wait, reload happened at step 3, before batch B. But the step-3 reload output showed "1 steps/frame" in the hint and "Max steps / frame" label — those came from batch B. Hmm, so the step-3 reload output was actually... Let me re-read.

Sequence of my calls:
- write index.html
- edit (5 blocks: placeholder removal, steps getter, probe flip, force attr, ok())
- edit (6 blocks → error, overlapping)
- edit (5 blocks: adaptive loop, label, speed oninput, hint span, ui.bar/note/hintSteps)
- edit (2 blocks: size 384, note 30)
- edit (1 block: tooSlow thresholds)
- browser reload → showed "grid 512×512", "every 20 frames", "1 steps/frame", "Max steps / frame ≤16"

So the reload showed batch-B changes but not batch-C (384/note 30)?! That's weird. Unless... the reload happened before the file writes were flushed? Or the browser tool's reload has a cache. Hmm, actually maybe the tool snapshots the file at some point.

Then `motion` showed the same 512/20 text with steps 945 — consistent with the page loaded at that reload.

So: reload may serve a cached version. To be safe, after each change I should reload twice, or add a cache-busting step. Let me test: run the probe now and see whether the DOM shows the probe page. If it shows the probe page, reload does pick up file changes (maybe there's a delay/write-race).

Let me reload and inspect.

The GL sim matches the CPU reference bit-for-bit (maxAbsDiff 0.0000). So the physics is correct and the seed does survive (hot=68 at n=400 at size 128).

So why did the app go to max V = 0? Because at 512 grid, the seed radius is 14.34 cells with U=0.2 at center, V=1.0 — and the dynamics at 512 differ (relative seed size smaller vs. diffusion length). Let me test the app's exact seed at size 512 in node... that's 512²×1000 = 268M updates ≈ maybe 60-120s in node. Let's test at 384 (the new size) for 1200 steps: 384²×1200 = 177M — also slow but let's try with a leaner loop. Actually node does ~10-20M cell-updates/sec here... let me measure quickly. Alternatively test at 256 and 384 for 600 steps.

Actually, in the browser run, at 930 steps max V was 0.0000 — total extinction. But my node test at 128 showed recovery at 300 steps (hot 56) and growth. Note the node "app current" test at size 128: r = max(6, 3.58) = 6 → hot 288 at 3000. The probe at 128 (same seed) showed hot=0 at n=100 then 48 at 200 → the pulse nearly dies then recovers.

At 512, r=14.34: the seed is bigger in absolute cells but the domain is bigger. The near-death dip at n=50-100 (hot 12→0) is concerning: the initial pulse with V=1.0/U=0.2 dies down to max 0.247 then recovers via... hmm, recovery at 200 means a spot survived at the ring.

But in the app, V went to exactly 0 and stayed. Possibly because in the app the stats were read at 512 grid where the extinction is complete. Let me test the app seed at 384 and 512 in node for ~1500 steps. Let me time a quick run.

Also, a much more robust approach for the seed: use a "softer" seed that reliably grows spots: e.g., U=0.25, V=0.5 flat in a disk of radius ~ size*0.03, or better: seed with several small spots? The spec says "seed a small central region with V and reduced U" — a single central region is required.

Let me empirically find a seed that at sizes 384/512 reliably grows (max V stays > 0.2 and hot count grows). Test candidates in node at size 384, 1200 steps. Let me first measure node throughput.

The app seed (V=0.9t+0.1 with U=0.45-0.25t → center V=1.0, U=0.2) dies at 384. The "U.25 V.50 r.030 flat" variant works well. Let me switch the seed to: U = 0.25, V = 0.5 in a flat disk of radius 0.030*size (with a small smooth edge maybe). "flat" (t=1 inside) is best: hot 60→244 growth.

Fix initialState: 
```js
const c=size*0.5, r=Math.max(5, size*0.030);
if (d<=r) { a[i]=0.25; a[i+1]=0.5; }
```
That's "seed a small central region with V and reduced U" ✓.

Restore the app file with this fix, plus keep size 384 (already edited in app.index.html.bak? The bak was made after all edits except... let me check: I copied index.html → app.index.html.bak AFTER the size-384 edit and the tooSlow edit. Yes, bak contains all edits. So restore from bak and patch initialState.

Also the splat strength: addV 0.55, subU 0.55 → similar to a working seed (V~0.5, U reduced) — good; injected regions will grow spots.

Let me do it fast.

The seed needs to change: use a flat disk with U=0.25, V=0.5, radius 0.030*size (min 5). That reliably grows spots at 384/512.

Also splat: keep addV 0.55 / subU 0.55 — that reduces U to ~0.45 at center of splat, V to 0.55 → similar to a working seed. Good (node test showed U.25 V.50 works).

Let me patch initialState now and verify in browser quickly.

Also raise the splat to match a working seed: uAddV 0.55, uSubU 0.55 → U drops to 0.45*... Actually u = s.x - 0.55*w*s.x → at w=1, u=0.45, v=0.55. Node test "U.35 V.55" worked (hot 240 at 1400). Good enough.

Now reload and verify: initial seed visible, evolution over time (hot count grows), click adds region, pause/resume, reset restores.

Note: reload may serve cached file — earlier reload did pick up edits (the 384 edit... hmm, the reload after the 384 edit still showed 512!). That's worrying. Let me check: after this reload, does the DOM show "grid 384×384"? If not, the tool caches; I'd then need another approach (e.g., touch a query param — not possible). Let's test.

Reload works and picks up edits (good — earlier confusion was just ordering). Seed visible (bright disk), steps/frame adapting (6 now).

Now verify: evolution over ~20s, then click to seed a separate region, then pause/resume, then reset.

Let me do a motion pass (4 frames ~2s each) to see evolution. Then click at (300,250) in stage coords → the canvas spans x 138..728, y 57..647 roughly. Click at (300, 250) inside the stage.

Let me first check evolution with motion, then click.

Evolution is happening: the disk became a ring (hollow), max V 0.34, hot 196, steps 444. It's evolving (ring formation) — the classic behavior. The step rate dropped to 1 step/frame because frames are 33ms (the harness caps rAF at ~30fps and the readback stalls). Evolution is slow but real; the ring is expanding.

Given the time limit (~4 min left), I should verify pointer seeding quickly and finish. Steps/frame=1 is a bit slow for the demo; the tooSlow threshold uses dtms>30 which is always true at 30fps cap in this harness. Let me change the ramp-up condition to be less strict: only decrease if dtms > 45 (i.e., below ~22fps), and increase if dtms < 40. That keeps more work per frame while staying responsive-ish. In this harness the frame rate is capped at ~30fps anyway (33.3ms constant), so the adaptive logic misreads the cap as "too slow". Fix: use the submit cost as the primary signal (real work), and only use dtms as a secondary guard with a higher threshold.

Let me set: tooSlow = submit > 14 || dtms > 60; rampUp = submit < 8 && dtms < 45 (with maxSteps cap). Under software GL, submit (draw call issue) is fast; the GPU work happens async, so submit won't reflect load... but the readback in stats() forces sync. Hmm. Use a smoothed measure of "sim+render+stats" cost? The simplest robust proxy: measure the time from the start of the frame callback to just after render (submit only) — under SwiftShader, drawArrays is likely synchronous-ish (immediate driver), so submit does reflect cost. Earlier, with 16 steps at 512, frames were 183ms, and my earlier submit-based adaptation never reduced → so submit was NOT reflecting cost (async). So submit is unreliable here.

Alternative: measure the *displayed* frame rate over a window and target ~30fps minimum, but the harness caps rAF at 30fps (33.3ms) — so dtms ≈ 33.3 even when idle. That means dtms-based control can't distinguish. Best proxy: use the elapsed time between the start of the frame callback and the *next* frame callback (dtms) minus the known cap? No.

Practical approach: use a moving average of "frame-to-frame" time measured with performance.now() across the rAF boundary, and target keeping it under ~50ms (20fps). Since the harness caps at 33ms, threshold 45-50ms gives headroom to ramp up to maxSteps=16 while still degrading if the machine can't keep up. Let me implement: tooSlow = dtmsAvg > 48 || submit > 20; ramp up if dtmsAvg < 40.

With EMA of dtms (alpha 0.2). At 33ms idle → ramps up to 16 steps/frame; if that pushes frames to 60ms+ → settles. That's better: evolution 16x faster in this environment.

Let me implement quickly and verify with a motion pass. Also I must verify pointer click. Time is short — do the edit, then one combined check: click, then motion (which shows both evolution and the seeded region), then pause/reset check via inspect.

Let me write the edit.

Now reload, then click at a point away from center (e.g., 300,250 is inside stage which spans x≈138-728, y≈57-647). Click at (250, 500) — lower-left area of the canvas. Then motion to see evolution and whether the seeded blob grows. Then pause/resume + reset check.

Let me do reload + click in one batch? They're dependent (must reload before clicking). Do reload first, then click, then motion.

Time is short (~3 min). Let me combine: reload (1 call), then click+motion.

Now click to seed a separate region, then drag, then motion to check effect. Do click at (250,500) and drag from (500,200)→(560,260)? Time is short. Let me do: click at (250,500), then motion (4 frames) to see both evolution and the seeded spot growing. Then pause/reset check via inspect.

I'll batch click + motion? They're sequential-dependent but the tool executes in order within one message? Instructions say make independent calls in same block; these are dependent (motion after click). But the harness runs them in order anyway. To save time, I'll issue click and drag in one call block, then motion.

The click at (250,500) in viewport coords → the stage starts at x≈138, y≈57, so the click landed at canvas-relative (112, 443) → uv (0.17, 0.69) → in the screenshot the bright blob appears at upper-left (250,178). That's because the display is vertically flipped relative to the pointer mapping (GL uv origin bottom-left vs. canvas top-left). The splat uses uv=(x,y_event) directly as texture coords, and the display shader maps vUv.y=1 → top of viewport. So the splat appears at y = size - y_event*size → flipped vertically.

I need to fix the flip so the splat appears under the cursor: pass v = 1 - y_event to the splat shader. Then texture row = (1-y)*size, and the display shows that row at canvas y = (1 - ((1-y)*size)/size) = y. Correct.

Also then probe should use v = 1 - y_event.

Let me fix: in uvFromEvent, return [x, 1-y] so everything downstream is in texture space (uv with v up). Then the display mapping: texture v=0 → canvas bottom. And pointer at top (y_event=0) → v=1 → canvas top ✓.

So change uvFromEvent to flip y. Then probe(lastPointerUV[0], lastPointerUV[1]) is correct as-is (both in texture space).

Also note the seeded blob at upper-left looks bright yellow-green (V high) — good, the splat works and is alive.

Now the important remaining checks: pause/resume and reset. Time is very short (~1.5 min). Let me make the flip fix and finish; I'll do a quick reload + reset check via inspect if time permits.

Actually — careful: the "cells V>0.25 = 184" and max V 0.35 means the central ring is still alive and the click blob is alive. Good.

Let me apply the flip fix now.

Wait: for the CPU fallback backend, the display draws array row 0 at the canvas TOP (drawImage of an ImageData where index 0 = top-left). In that case texture space v=0 is at top, so flipping y would be wrong for CPU. Handle: flip only for WebGL. Let me make uvFromEvent conditional on sim.kind === 'WebGL2'.

Also the probe for CPU: uses u,v as row-from-top. With the flip applied only for GL, CPU stays consistent.

Quick edit.

Now verify: reload, click, check the splat lands under the cursor; then pause and reset and compare center checksum to initial.

I have very little time. Do: reload, then click at (250,500) and inspect (single call each). Then final: click Reset and compare center value.

Let me batch reload first (must be before click).

Baseline for reset comparison: initial "center 32² mean V = 0.16050", max V 0.4941, hot 424, mean V 0.00111.

Now: pause → click → check stats frozen → resume → click → check stats change → reset → compare center to 0.16050.

Time is essentially up. Let me do a final combined check efficiently: click Reset and inspect (verifying the restored seed equals 0.16050), since reset is the key remaining item. Do one call: click Reset button at (614,107)? The Reset button is at approx x=614, y=107 in the panel. Then inspect.

Actually let me do the pointer-position fix verification (click in canvas) AND reset in the remaining calls. Priority per the task's verification list: reset comparison. But the flip fix is unverified visually.

I'll do: click at (500,500) in canvas (should now appear lower-right), then click Reset, then inspect. Two dependent calls in sequence... I'll issue the canvas click now, then a final reload-free inspect+reset.

Given time, do one call: click at (500,500). Then one call: click Reset (614,107). Then done + cleanup temp files.

The click at (500,520) produced a bright blob at exactly (500,518) — pointer mapping now correct, and the injected V region is alive (bright white = high V). The central seed evolved into a ring/square structure.

Now test Reset → should restore center checksum 0.16050 and the initial look. Click Reset button at (614,107).

My click at (614,107) landed inside the canvas region? No — (614,107) is in the CONTROLS panel... but the screenshot shows a new bright blob at top (~615,107 in page coords → inside the stage? The stage spans x 138..728, y 57..647. Yes! x=614, y=107 IS inside the stage (the panel starts at x≈855). Oops — the screenshot coordinates are scaled: the screenshot is 1200x800 but the actual viewport maps... the DOM panel appears at x>855 in the screenshot, which is beyond the 1200 viewport? No, the screenshot is the 1200x800 viewport. So the controls panel is at x≈860-1170 in the screenshot, and my click at x=614 hit the stage. So the Reset button is at screenshot x≈1001, y≈107.

Anyway, this click acted as a pointer splat (proving pointer mapping again, at the correct location under the cursor) and the probe readout now shows values: "probe U / V = 0.075 / 0.598" — wait, that's from the earlier click at (500,520)? It shows live probe readback works.

Now I'm nearly out of time. I need to verify Reset. Click at (1001,107) for the Reset button.

Note the state: sim steps 2249, center 0.11639 (evolved). After reset, center should be 0.16050 and steps 0.

Let me click Reset at (1001,107).

Reset works: sim steps → 0, max V = 0.5000 (exactly the seed value), hot = 424 cells (the seed disk), the canvas shows the pristine central seed disk again, and the note updated. The center checksum reads 0.20703 vs. the initial-load 0.16050 — because the initial reading was taken after 16 steps had already run (V had partially diffused), whereas reset measures at step 0. So the restored state matches the true initial condition exactly (max V 0.5 = seed value, hot 424 = seed disk area π·11.5² ≈ 415 ✓).

Everything essential is verified. Clean up temp files and finish. Time is up — do the cleanup and report.

Pause/resume: I verified the button label/state indirectly? I did not explicitly test pause. The code path is simple (setPaused + loop guard) but unverified. I should note that. Actually I have seconds left; skip further testing and report honestly.

Clean up: remove probe2.html, app.index.html.bak, /tmp/gs*.js (gs files are in /tmp, fine to leave, but the job dir should just have index.html).

Wait, "bytes 30565" but "30661" from ls — because of the regex count? whatever. No external refs (0 occurrences of http(s)://). Good.

Hmm, but the file lost ~100 bytes? No, ls says 30661, node read 30565 (length in chars vs bytes with multibyte UTF-8 chars like ·, ²). Fine.

Final check: does the file still contain the `hint` span and the reset note? Yes.

Done. Report.