Fireworks · deepseek-v4.1-flash

Let me review the Shell ascent physics. vy = -sqrt(2*0.28*dist) - rand(1,3). This makes it reach around target. vx = (tx - this.x)/(|vy|/0.28) — time to peak approx |vy|/0.28. Good enough.

Let me test in browser.

The demo shell should have fired at 500ms. The screenshot is at frame 1 probably immediate. Let me wait and inspect via motion. Actually the demo fires after 500ms. Let me capture motion frames.

I need to check the shell ascent. There's a concern: `window.addEventListener("load", ...)` — if load already fired before script ran? Script is inline at end of body, so load fires after. Fine.

Let me trigger a click and sample motion.

The demo shell is rising near the right side and doesn't seem to burst. It appears stuck at bottom right — only rising a tiny bit over 1.7 seconds. That suggests the shell is barely moving. Why? Let me check the physics.

vy = -sqrt(2*0.28*dist) - rand(1,3). For dist ~ 700px, sqrt(2*0.28*700)=sqrt(392)=19.8. So vy ≈ -21. Gravity 0.28 per frame. Time to peak ~75 frames ~1.25s. Should rise significantly. But it seems only rising ~30px over 1.7s in the images (from y~730 to ~700). Hmm, the trail marks are at bottom right, positions around 815,733 and 813,700. Wait these are trail marks left behind, not the shell itself.

Actually the demo launched at W*rand(0.3,0.7) which is 360-840, H*rand(0.22,0.4) = 176-320. The shell at frame 4 (1.7s) should be near target. But I only see trail marks low. Hmm.

Wait, maybe the issue: the shell explodes when `this.y <= this.targetY`. If targetY is 176-320, shell needs to rise from 800. But all I see are trail dots near bottom. Maybe the frame rate is low because of motion capture, dt clamped to 3 max. Still should progress.

Hold on—the trail marks are gray/dim and appear at y=733 and y=700. The shell itself is bright. The bright shell may have already burst and the particles faded. Between frame 1 and frame 2 (+566ms), demo launched at 500ms. Actually the motion frames are 0,566,1137,1694. So at frame 2 the shell just launched.

Hmm the trail marks barely move. Let me inspect actual JS state. I can't directly query variables since they're in closure. Let me add a temporary debug hook, or better, evaluate via console? The browser tool doesn't expose eval. Let me use inspect to see canvas only.

Actually maybe rendering is fine but the shell velocity is small. Let me reconsider: `this.vx = (tx - this.x) / (Math.abs(this.vy) / 0.28);` The time to peak = |vy|/0.28 ≈ 75 frames. vx small. Fine.

But wait, gravity uses dt normalized. In update, `this.vy += 0.28 * dt; this.x += this.vx * dt;`. At 60fps dt=1. vy=-21, moves -21px/frame. Over 4 motion frames spanning 1.7s ≈ 100 frames, should rise 800px and burst. So why only 30px?

Unless requestAnimationFrame isn't running at 60fps in this environment. The motion capture might pause rAF. But the trail marks progressed only slightly too. Actually trail timer spawns every 0.6 dt. Over 1.7s should spawn ~170 marks... but MAX_TRAIL 420. But we only see 2-3 marks. So updates are barely happening. That means rAF is being throttled/paused between screenshots, OR `dt` is tiny.

Another possibility: The demo launched at 500ms but the images are frames 2,3,4 each +~560ms. At frame 2, shell just appeared at bottom. At frame 4, still bottom. Very slow.

Wait — maybe the issue is that the screenshot "motion" captures at specific times but rAF runs normally. Let me think about `last`: initialized `var last = performance.now()` at script parse. Then frame uses dt=(now-last)/16.67 clamped to max 3. First frame dt could be 3. Fine.

Hmm, could the shell have exploded already and these are leftover trails from the burst location? The burst would be at target (right side, y~176-320), not bottom. The dots at bottom right suggest shell trails near launch.

Let me just add a debug: expose state to window for inspection. But I can't eval. Actually I can add DOM text with debug info and inspect. Or simpler — write a quick test harness that runs the logic in node. But real concern: does it work visually?

Let me take a screenshot after waiting a bit more, and also click to test. Let me do reload then wait via multiple screenshots.

Actually, let me reconsider the demo timing. `window.addEventListener("load", ...)` — in a sandboxed iframe, if the load event already fired before this listener attached? Inline script at end of body executes before load, so no.

Let me check: maybe the motion capture only renders a few frames and the page is paused otherwise. The "motion" tool likely captures frames from an animation but the page may run in real time. The trail positions moving from y=733 to y=700 (33px) over ~1.1s between frame2 and frame4. That's 30px/s. Way too slow.

Oh! I bet the bug: `this.vy = -Math.sqrt(2 * 0.28 * dist) - rand(1, 3);` with dist = Math.hypot(tx - this.x, ty - this.y). Fine ~20.

Wait, but `Shell` sets `this.x = tx + rand(-W*0.05, W*0.05)` and compute dist using this.x. Fine.

Hmm, let me actually instrument. I'll add a debug overlay showing shell count, particle count, and shell y. Then inspect/screenshot.

Actually maybe easier: run the physics in node quickly to confirm. But the visual discrepancy is about rAF timing, not physics.

Let me check the frame loop: `ctx.fillStyle="rgba(5,6,15,0.28)"; ctx.fillRect(...)`. Then trails drawn. Then shells update and draw. Good.

Let me add debug text and use inspect.

The div edit failed due to formatting. Let me just do the debug hook, and add a periodic dbg update. Actually simpler: expose window.__fw and then... I can't eval. But the `inspect` action returns DOM. If I write state into DOM I can read it.

Let me add the debug element properly.

Now let me click somewhere and inspect state over time. First click at, say, (400,250). Then screenshot after a short wait. The browser tool doesn't have a wait action; each call returns state. Let me click and then inspect.

Very revealing! After the click at (400,240), the debug says shells=0 parts=0 trails=10. But there's a bright shell at (365,745) near bottom. Wait, shells=0 means the shell was already removed. But the bright dot at (365,745) — that's odd. And trails=10.

Hmm wait, the debug interval runs every 100ms, and the click screenshot is taken right after. The shell at (400,240)? No, the bright dot is at (365,745). That looks like a shell just launched from click at x≈400 (with jitter) at y=745. But shells=0... 

Oh! I see. The screenshot and DOM inspection may be at slightly different times than the setInterval update. But shells=0 contradicts a visible shell.

Wait, actually maybe the click triggered a shell, and by the time of DOM read the shell already exploded? No, that can't be within ~0ms.

Actually more likely: the `pointerdown` launched a shell, and the debug read happened before the next setInterval tick (which shows stale state from before the click). But the visible bright dot is the newly launched shell. trails=10 includes old demo trails.

But wait, where's the demo shell? It seems it never exploded — trails=10 persists. Earlier demo shell was at bottom right. Now there's a green trail at x=713 bottom. Hmm.

Let me take another screenshot to see progression.

Interesting: trails count went 10 -> 20, shells=0, parts=0. The bright dot is at x≈372 now (was 365), y≈733 (was 745). So the shell is rising slowly and NOT counted in `shells`? But it should be. shells=0 is strange.

OH. I think I understand: the debug shows `shells.length` which is 0, but there's clearly a shell being drawn. Unless... the bright dot is actually a particle? No, particles=0.

Wait, could there be TWO timers? Actually the thing at bottom: earlier frame showed bright dot at (365,745). Now at (372,733). Moving right and up slowly. That's consistent with a shell launched from click x≈400: x starts at tx+jitter, vx positive/negative.

But shells=0... Let me reconsider. Maybe `shells` variable in the debug closure refers to a different array? No.

Hmm, actually maybe the bright dot is the shell, and shells.length is 0 because the shell exploded and got removed, but then... no, only one shell.

Wait, actually maybe the setInterval callback throws an error because `shells[0]` when shells.length is 0? No, guarded.

Let me reconsider: earlier at reload, shells=0 parts=0 trails=0. Then demo launches at 500ms. The click screenshot showed the bright dot. If the click created a shell, shells should be 1. Unless click event didn't fire and the bright dot is the demo shell, which is still alive. And shells=0 means... the demo shell exploded? But no particles.

Something is fundamentally off. Perhaps the debug readout is stale because setInterval only ticks when the iframe is active, and the screenshot is captured during a paused moment. The `dom` string may come from a cached snapshot.

Alternatively: Maybe requestAnimationFrame is NOT running (throttled) most of the time, and instead the rendering we see is from the animation tool's forced frames. The dev tool "motion"/screenshot might drive rAF. The setInterval would still run though.

Let me test more directly: add a counter that increments per frame and track. Let me expose more: frame count, and log. Actually, let me reconsider the possibility that there's a JS error in frame() that halts the loop after the first frame. If frame threw, requestAnimationFrame wouldn't be scheduled again. But then the shell dot wouldn't move at all across screenshots. It moved from 745 to 733 over one screenshot. That's 12px. Could be one frame of movement (vy~-12? no vy~-20).

Hmm, actually maybe the loop IS running but very slowly — like a few frames per second — because the environment throttles rAF. Then dt clamped to 3, so each frame advances 3/60=0.05s. Shell moves 20*3=60px? No, dt applies to vy: vy += 0.28*3. Position += vy*3 = -60px per frame. Over screenshots maybe 1 frame.

The observed 12px movement doesn't match 60px.

I'm overanalyzing the harness. Let me instead write a deterministic test in Node that simulates the physics to validate the logic, and separately trust the visuals. But the requirement says run in browser. Let me make the debug more informative: include frame count and last dt, and display. Then take screenshots over time.

Actually, a cleaner approach: the browser tool's screenshot likely waits for a stable frame. Let me just click and immediately take motion to capture the sequence. Earlier motion showed the shell rising slowly near right. 

Let me reconsider the physics bug: `this.vy = -Math.sqrt(2 * 0.28 * dist) - rand(1, 3)`. Note gravity in update is `0.28 * dt`. Good. But `dist = Math.hypot(tx - this.x, ty - this.y)`. If target is near bottom (ty close to H), dist small, vy small. For click at y=240, dist ~ 560+, vy ~ -18.

I wonder if the real problem is that `dt` is tiny because `performance.now()` advances but frame is called rapidly? No.

Let me just add framecount and dt to debug.

Also let me add a visible representation for particles maybe. But first reload and observe frame count over screenshots.

f=7 at reload, meaning only 7 frames rendered so far. rAF is heavily throttled in this headless environment (only runs when capturing). So the shell rises slowly relative to wall-clock screenshots because frames only happen during captures. That explains everything: the demo shell takes many frames to rise, and the browser tool only renders a few frames per screenshot.

So the visual behavior is fine; the harness just doesn't run rAF continuously. To verify properly, I should drive frames manually or use motion which captures multiple frames.

Given this, the shell physics is correct. But for the demo to be visible "on initial load" in a real browser it's fine.

Let me verify burst and particles by clicking and using `motion` which captures 4 frames, or repeatedly screenshot to accumulate frames. Let me click at a low target so it bursts quickly (many frames needed anyway).

Actually, since rAF only runs during captures, I can force many frames by taking many screenshots. Or better, temporarily add a way to step frames. Let me add a debug function to advance N frames synchronously? The frame loop uses rAF. I could add `window.__step = function(n){...}` but can't call it.

Alternative: change demo to launch and let me use motion repeatedly. Let me click at (400, 500) so target is lower (dist ~300), faster burst (~sqrt(2*.28*300)=13, time ~46 frames). Still many.

Better idea: Instead of relying on rAF throttling, I'll reduce the verification burden by making each captured frame advance more. But that changes product behavior.

Actually the cleanest: use the browser `motion` action which seems to capture 4 frames ~560ms apart, but those frames correspond to actual rAF calls. Still few.

Hmm, let me think about whether rAF is really throttled or whether the debug setInterval runs but frame count stays low. f=7 after reload + screenshot. The screenshot is taken immediately after reload, so maybe only 7 frames happened in the ~100-200ms since load. That's plausible at 60fps? 7 frames = 116ms. And the demo hasn't fired (500ms). So actually rAF may be running at normal speed, and the screenshot was just early! The "+0ms" frame timing might not reflect real elapsed.

When I clicked earlier, it was presumably some time after reload, so demo should have fired. But screenshots showed trails increasing 10 -> 20 and shell near bottom. Hmm.

Let me do a better test: reload, then take several screenshots in sequence (each call takes real time), and observe if particles appear. Let me click at a reachable point and take multiple motion frames.

Let me click at (400,300) then call motion.

Actually wait — earlier click produced trails=10 immediately, and the bright shell near bottom. That click was at (400,240). If rAF were fast, the shell would race upward. The screenshots showed it barely moving. But between those two screenshots real time passed (maybe a second). So rAF is throttled to only run during capture operations.

The `motion` action captured 4 frames. So motion drives rAF 4 times. So to advance the sim, I need many motion/screenshot calls. That's expensive.

Solution: I'll add a temporary debug "fast-forward" that runs multiple simulation substeps per rendered frame, but only for testing. Actually simpler: temporarily set a huge dt or loop inside frame. But then the real product changes.

Alternative approach: Make the application robust to low frame rates by using real elapsed time (already does, dt clamped to 3 = 50ms max). Clamping to 3 means at low fps it lags. In real browser at 60fps it's fine.

I think the functionality is correct. But I should still verify the burst visually. Let me reduce the demo's target distance / make launch faster so few frames suffice? No.

Let me just temporarily add an auto-test mode via URL hash that runs the simulation headless using a fixed timestep and renders, then I can inspect the final state. Actually I can add `window.__tick(n)` and trigger it via a button? I could add a hidden debug button that runs 200 simulation steps instantly, then screenshot shows the burst. That's a legit temporary debug.

Even better: I'll temporarily change the main loop so that if a global flag is set, it runs multiple substeps. But I need to set the flag from outside. I can't.

I can add a temporary debug button "FF" that when clicked advances. Clicking is available. Let me add a temporary fast-forward button that calls a function running e.g. 120 steps of update logic then draws. Let me refactor update/draw into a step function taking dt, and add a debug button.

Actually, let me reconsider — maybe I'm wrong and the frame loop runs at 60fps during screenshots. Let me just take a screenshot, wait (the call itself, then another screenshot) and check fcount growth.

f=319 now, and shells=0, parts=0, trails=0. The demo shell already came and went (burst and particles faded) between screenshots. The screen is empty. So rAF is running at normal speed in real time. Great.

So my earlier confusion: the click screenshot was taken right at launch; subsequent screenshots happened after the shell already burst and faded. The bright dot near bottom was a freshly launched shell and the debug state was stale/at a different instant.

So things work. Now let me verify the burst visually. The problem is screenshots are too slow to catch the burst. I need finer timing. The `motion` action captures 4 frames quickly; maybe I can click then motion to catch ascent/burst. But click and motion are separate calls with delay.

Better: temporarily add a test hook that triggers a launch and captures? I can't run JS directly.

Approach: Add a temporary "Demo/Test" that, when I click the sky, launches a shell from click location. Then immediately call `motion` — but the shell will have advanced ~1s by then and may have burst already (ascent takes ~1.1s). Motion captures 4 frames over ~1.7s. So it might catch the burst.

Alternatively, slow the simulation temporarily for verification. Or make the shell travel a shorter distance (click low), so burst happens in ~0.5s. Let me click at y=550 (target closer), then motion.

Also I realize the demo at load: by the time verification screenshots happen it's gone. That's fine.

Let me do: click at (600, 520), then motion.

Actually to really see it, let me temporarily add a debug mode: pressing 'd' or a button spawns a shell and also freezes time scale to 0.3. Hmm, complexity.

Simplest reliable verification: Add a temporary query-string/hash test that, on load, launches a shell with reduced gravity so it rises slowly, then screenshot. But again timing.

I think the best is to add a temporary debug button "Test" that launches a shell and also logs positions to DOM, and take motion right after clicking. Let me just try motion after click and see.

Actually, let me add a hook that makes a shell explode immediately plus emit particles, so I can verify particle rendering/fading clearly. But that doesn't verify ascent.

Let me try a controlled test: temporarily set the demo to launch repeatedly every 2 seconds so screenshots always catch something. That's actually a reasonable feature but not required. Hmm, "Launch one demonstration shell automatically on initial load" — only one.

Let me just do a click then motion and hope to catch the ascent. Click at (600,400).

But there's a subtlety: motion may start capturing immediately after the click and run 4 frames 560ms apart = 1.7s. Shell ascent ~1.2s then burst. So frames 3-4 catch burst/fall. Good.

The click at (600,420) launched a shell at x≈622 (jitter), y=759, vy=-16. DOM says shells=1 y=759 vy=-16. The screenshot shows bright shell at (623,750) plus a trail at (450,727). Interesting: there's a dark blob around (450,720) which may be a fading trail cluster from previous demo burst.

Now let me capture motion to see ascent and burst.

Motion frames: frame1 f=1015 (trails=10, shells=0), frame2 f=1045, frame3 f=1081, frame4 f=1111. Between motion start the shell was already gone (shells=0). The screenshots show fading dark blobs. It seems the burst happened before frame1 or the particles are very dim. Actually at f=1015 trails=10, shells=0, parts=0 — so the shell already burst and particles already gone?! But the click was just before motion. Frame counts: click screenshot f=889, motion frame1 f=1015 — that's 126 frames ≈ 2.1s later. So yes, by the time motion began, the burst (~1.2s) and particles (~2s) mostly done.

The motion tool takes a moment to start. The dark blobs near bottom are trails that are barely visible because trail alpha 0.28 and color. Hmm they look very dark — maybe because the trails fade and only show faint.

The core issue for verification: I can't catch the burst with the screenshot tool because it starts ~2s late. I need to make the demo visible in screenshots. Let me temporarily increase shell flight time (slower) OR better, add a temporary "slow motion" scale.

Actually, a good approach: temporarily set the simulation to loop demo shells continuously (auto-launch every 1.5s) so that any screenshot catches an ascent/burst/particles. I can do this as a temporary test and remove it after. But the required behavior is a single demo on load. Continuous might actually be fine as an extra, but let's not.

Alternatively, I can make shell ascent take a random 2.5-4s and particles live 3-5s during testing. But better: I'll add a temporary auto-loop, verify, then remove.

Let me add temporary auto-launch every 1200ms and also expose particles. Then screenshot multiple times to catch mid-burst. Let me also lower the particle fade speed? No.

Actually the screenshots seem to catch the scene well enough once shells are frequent. Let me do it.

Let me edit: add a temporary setInterval launch every 1300ms.

The dbg element seems missing from DOM now — the dom string only shows "Clear\nclick the sky...". Wait, dbg text should appear. Maybe it's empty at first inspect. Fine.

Now take a motion capture which spans 1.7s and with launches every 1.3s should catch multiple shells/bursts.

Now I see ascending shells (green and white) with trails. But I still haven't caught the burst visually. Shells=0 in all frames but bright ascending dots. This confirms debug state is read at a different time (setInterval tick) than screenshot.

The shell ascent is visible. But the shell seems to rise very slowly over the 1.7s capture — from y~790 to y~740 only. That indicates the shell ascent is much slower than intended in real time? Let's compute: shell launched at target H*rand(.22,.4) ~ 176-320. Distance ~ 600. Speed ~ -sqrt(2*.28*600)=-18.3, minus 1-3 -> ~-20. At 60fps, that's -20px/frame = -1200px/s. It should reach target in ~0.5s, not 1.7s.

But the frames show only ~50px movement over 1.7s. That means while capturing, rAF runs at only ~? Between f=126 and f=162 there are 36 frames over 555ms => ~65fps. So 36 frames * 20px = 720px. But visually it moved ~50px. Something's inconsistent.

Hmm wait, in frame 1 f=406... no. Frame2 f=162, frame3 f=182, frame4 f=198. Actually f increments ~20 per 555ms => ~36fps. Still 20 frames * 20px = 400px. But observed ~50px.

Unless the shell just launched near the end of each frame's timing, i.e., we're seeing different shells each frame. Indeed launches happen every 1.3s. Each frame might show a different newly launched shell at similar position. That explains why they all appear low: we keep catching fresh shells.

But then where are the bursts? Shells explode at target ~y=250. We should see bursts high up in some frames. None visible. Hmm. Unless the explosion happens and particles are very dim by the time of capture. But shell flight ~0.7s and capture interval 0.55s, should catch some.

Wait, maybe the shell never reaches the target because of a bug! Let me re-examine. `if (this.y <= this.targetY || this.vy >= 0 ...)`. targetY is H*rand(.22,.4) e.g. 250. Shell starts y=808, vy=-20. After n frames: y = 808 -20n + 0.14n^2. It reaches 250 at... solve 0.14n^2 -20n +558=0 => n=(20-sqrt(400-312))/0.28=(20-9.38)/0.28=37.9 frames. So in ~38 frames (0.63s) it bursts. Good.

So bursts should happen every 1.3s and be visible. Yet screenshots show only ascending shells low. Perhaps I'm capturing during the ascent each time due to 1.3s cadence beating with 0.55s frame interval. 1.3/0.55 ≈ 2.36 periods. So phases vary. Some frames should catch bursts.

Let me take several plain screenshots in a row to sample different times. Actually, let me just use the motion with longer observation: call screenshot multiple times.

Alternatively, the burst particles may be rendering but very small/dim. The particle gravity 0.085 and initial speed 1.2-6.4 px/frame. Over lifetime 1.1-2.4s. They spread ~ up to 6.4*66 = 422px. Should be visible.

Wait, maybe there's a rendering issue: particles are drawn with shadowBlur and alpha, but the motion-blur fill `rgba(5,6,15,0.28)` each frame. Should be fine.

Let me take more screenshots to catch a burst.

Still catching shells low. This is suspicious. Maybe the shell never explodes and just keeps going up? But shells=0 in debug. Hmm.

Wait, look at frame: shell at x≈667, y≈727 ascending with trail dots below. Next screenshot earlier had shell at ~955. These are different shells.

The consistent low position across many captures suggests each capture happens shortly after a launch. With launch every 1.3s and capture cadence irregular, maybe.

But I'm worried the shell ascent is actually slow in real time. Let's test: reload, then immediately screenshot twice and compare within known frame counts. Actually debug gives f counts. Between reload (f=7) and next screenshot f=319 => 312 frames. If that was ~5s, that's 62fps. Good.

Let me directly measure: capture the debug state showing y in the DOM! The debug DOM is updated by setInterval every 100ms with shells[0].y. In screenshots, the DOM text shown is from the previous interval tick, but it should usually show a shell if alive. Yet it always shows shells=0. The screenshots show a shell though. That means the DOM snapshot is stale (from a moment with no shell) OR the shell in the image is a particle.

Hold on! Maybe what I see ascending is not a shell but... no, particles only exist after burst.

The debugging is getting confusing because DOM and image are from different instants. Let me change the debug to draw text ON the canvas each frame, so the debug info is guaranteed to be in the same frame as the visuals. I'll use canvas fillText in the frame loop.

Let me do that.

That edit did nothing meaningful. Let me instead add on-canvas debug drawing at the end of frame. Let me view the relevant part.

There are two `requestAnimationFrame(frame)`? Line 238 inside frame, line 252 after. Let me look at lines 230-270.

Now let me add on-canvas debug text at the end of frame before requestAnimationFrame. This will be drawn on canvas and visible in screenshot, synchronized with the frame. Also draw shells count etc.

Note: drawing debug after particles. Let me insert.

But `fcount` and `lastDt` are declared after `requestAnimationFrame(frame)` at line 253 (var fcount=0,lastDt=0). Due to hoisting, var is hoisted but assignment happens after first frame call. First frame uses fcount undefined -> NaN. Minor. But since the first frame executes asynchronously after the script finishes, fcount=0 already assigned. Fine.

Now reload and take a screenshot. With on-canvas debug text synced, I can see whether shell y reaches target and explodes.

On-canvas debug shows f=4 shells=0. Now let me take screenshots to observe. Let me click at (500,350) and then take a screenshot right away, and repeat.

Actually, to catch the burst, let me increase the capture frequency by calling screenshot repeatedly. Each call seems to happen quickly (the frame count jumps a lot though). Let me call screenshot several times in one message (independent calls). But tool calls in one block run in parallel? They might be sequential. Let me just do a few.

Actually, a cleverer test: since shells auto-launch every 1.3s and each lives ~0.6s ascent + burst, a burst exists for most of the time. Yet screenshots never show bursts. This strongly suggests a bug where particles aren't rendering, OR the shell never explodes high and instead explodes at launch (particles near bottom, dim).

Wait, earlier I saw dark blobs near bottom center that looked like they could be faded particles. Let me look again at the screenshot with f=486: there was a faint smudge near (670,720). Could be particles at launch point? No.

Let me reconsider: shell explodes when `this.y <= this.targetY`. But targetY passed from demo = `H * rand(0.22, 0.4)` ≈ 176-320. However, look at `Shell(tx,ty)`: `this.vy = -Math.sqrt(2 * 0.28 * dist) - rand(1,3)`. dist = hypot(tx-x, ty-y). This gives apex energy for exactly reaching ty under g=0.28. But actual apex is slightly higher due to extra rand(1,3) velocity, so it reaches targetY. Good, should explode.

Unless dt is not 1: the on-canvas debug should show dt now? I removed dt from canvas text. Let me add it. Actually f counts increment ~ per frame.

Hmm, in the screenshot f=4 and shells=0 — right after load. Let me take a sequence of screenshots to see f and states.

Let me do 3 screenshots in parallel.

Now with on-canvas debug I can read: "f=268 shells=0 parts=0 trails=20 y=-1" but there's a red shell at (592,730). Wait the text overlapped/smeared showing multiple. The shell is visible but shells=0. Contradiction again! Unless the text is from a previous frame (the canvas has motion blur, so old text smears and the new text is drawn over). The text region shows redraws, hard to read. But it says shells=0 while a shell renders.

OH WAIT. I think I understand now! The bright dot might be a PARTICLE from a burst, and `parts=0`. No.

Actually, maybe the bright dot is a shell but `shells[0]` is a different shell? No, shells count 0 means none.

Let me think about `explode`: it creates particles and trails, then sets shell.dead. Not relevant.

Hmm, could it be that `launch` pushes to `shells`, and `demo` calls launch, but `shells` is also the name of the local in the frame loop? Let me check: in frame, `for (var s = shells.length-1; ...)`. `shells` refers to outer. Fine.

Wait — maybe the bright dot is a shell drawn by `draw()`, but by the time the debug text is drawn (at end of same frame), the shell got removed because it exploded during update, and its particles... but then parts should be >0. Unless the particle array was cleared? No.

Actually! Explosion happens inside `Shell.update`, which is called in the loop. If the shell explodes, `shell.dead=true`, then it's spliced, and `explode` pushed particles. So parts>0. Unless... explode creates particles with `new Particle` and pushes to `particles`. That should be >0.

Unless the shell's `draw` is what we see but its update is not being called because... no.

Let me reconsider: maybe the bright dot is a trail mark, not a shell. Trails are drawn with alpha 0.28*life and radius up to 2.2 — too dim/small to be the bright glowing dot. The bright dot has white core and glow — that's Shell.draw. So a shell exists.

Is it possible that the on-canvas text "shells=0" is stale because canvas is cleared with semi-transparent fill and old frames remain? The text is drawn every frame at same spot; old text fades but new is bright. In the screenshot, the green text looks bold/overlapping e.g. "f=268... shells=0". It's current.

This is a genuine mystery. Let me look at the code again for a subtle bug: maybe `demo` is defined and `launch` pushes to shells, but `shells` array referenced in `Shell.update`? No.

Hold on, maybe there are TWO canvases/frames? No.

Wait, maybe the debug text `shells.length` is evaluated in the frame loop BEFORE the shell gets added by the setInterval, and the shell visible is drawn by a *separate* rAF? No.

Let me reconsider the possibility that the bright dot is actually the mouse cursor? No.

Actually — could the bright dot be a "trail" from the shell with life close to 1 and radius 2.2, drawn with shadow? No shadow on trails.

Let me just directly print shells.length and also shells[0].y in the same text, plus a marker at shells[0] position. Alternatively, I realize the discrepancy might be because `fcount` and the arrays are fine but the screenshot catches a frame in the middle of drawing: text drawn at end shows state after updates. A shell that existed at draw time would have been drawn and counted. Unless it was drawn, then... no.

Hmm, let me reconsider: maybe the bright dot is the demo shell and the setInterval debug shows 0 because those shells fired and died between the last text draw and the setInterval? The canvas text is drawn each frame though.

Let me add a distinctive marker: draw a magenta circle around shells[0] if present. If the bright dot isn't circled, it's not a shell.

Alternatively, let me simplify: I'll log to console. Console output is captured. Add `console.log` on launch/explode and every frame state. That will disambiguate timing. Let me add console logs in launch and explode and at frame 100. Actually console output shown is only warnings; maybe logs appear too.

Let me add logging.

Console only shows the warning; my console.log lines aren't shown, maybe the tool filters. Let me just take another screenshot after a couple seconds to see logs. Actually the console array is from the page load moment. Let me trigger a click and see console.

Hmm, maybe console.logs are captured. Let me click then look.

Now it's clear! Console logs:
- LAUNCH 445 181 (demo)
- EXPLODE 393 731 parts=0  ← explosion at y=731! Not at target y=181!
- LAUNCH 496 217
- EXPLODE 448 731 parts=0

So the shell explodes at y≈731, near the bottom, not at the target. And parts=0 because explode logs before particles are added (log is at top of explode). That explains no visible particles high up — the burst happens at the launch point, and particles spread from there but quickly? Actually parts=0 in log is pre-push, fine.

Why explode at y=731? Because update checks `this.y <= this.targetY` OR `this.vy >= 0`. At launch, vy is negative, so not >=0. y starts 808, target 181. It should rise. But it explodes at 731 after only ~77px. That means `this.y <= this.targetY` triggered... at y=731 > 181, false. Or `this.vy >= 0` true at 731? That means vy became positive (falling) almost immediately. And life>3.2? No.

Wait, EXPLODE at 393 731: the x moved from 445 to 393 (leftward 52px) and y from 808 to 731 (up 77px). If vy became >=0 quickly, then at the top of a tiny arc. That means |vy| is small. Why?

Compute vy: dist = hypot(tx - x, ty - y). tx=445? Actually demo passes tx and ty; Shell sets this.x = tx + jitter. Then dist computed with target ty=181. dist ≈ hypot(small, 808-181=627) ≈ 627. sqrt(2*0.28*627)=sqrt(351)=18.7. vy≈-20.7. Not small.

But wait — the `dt`! Console says dt=1.00 in DOM. Fine.

Hmm, but EXPLODE at y=731 suggests it exploded after a few frames. Let's compute after 4 frames with vy0=-20.7, g=0.28: vy after: -20.7+0.28n. y = 808 -20.7n +0.14n^2. At n=4: 808-82.8+2.24=727. vy=-19.6, still negative. Not >=0. So why explode?

Unless `this.vy >= 0` is true from the start — maybe rand(1,3) and sqrt produce... no, negative.

OH! I see it. The explode condition includes `this.life > 3.2`, but life accumulates dt... no.

Wait, maybe the explode at y=731 is from a *different* shell than the launch logged. The LAUNCH 445 181 then EXPLODE 393 731. The x 393 could be that shell after moving left. But why y=731?

Let me recheck Shell.update:
```
this.vy += 0.28 * dt;
this.x += this.vx * dt;
this.y += this.vy * dt;
this.life += dt;
...
if (this.y <= this.targetY || this.vy >= 0 || this.life > 3.2) { explode... }
```
Hmm this looks right. Unless `this.targetY` is somehow 731? No, demo passes ty=H*rand(.22,.4)=176-320.

Wait, could `H` be wrong at demo time? H is set in resize to window.innerHeight = 800. Fine.

Let me reconsider the ordering: Is `explode` being called from a *falling* shell that launched long ago and reached apex and came back down to 731? The LAUNCH 445 181 was logged, then EXPLODE 393 731. If the shell reached apex at 181, explode should trigger at apex (y<=181 or vy>=0), not at 731. Unless the `if` check is somehow not reached... 

Unless `explode` is called but the shell continues (dead only set after). No, dead=true set right after.

Hmm, wait. Actually maybe the launch log 445,181 is the FIRST demo launch, and it exploded at 393,731?? That would mean the shell went UP toward 181 but exploded at 731 on the way up. Contradiction unless targetY isn't 181.

OH WAIT. I bet the issue is variable shadowing / the `demo` function: `launch(W * rand(0.3, 0.7), H * rand(0.22, 0.4))`. ty is fine.

But look at the click: LAUNCH 500 300 (target y=300). No explode logged yet in the snapshot. Then presumably it will explode at some y.

The other two: LAUNCH 496 217, EXPLODE 448 731. Again explodes at 731.

Pattern: explosions always at y≈731 regardless of target. 731 = ? H=800. 800-731=69. Hmm. Interesting: maybe `this.vy >= 0` becomes true after exactly 69px rise because vy is tiny? If vy0 ≈ -? For vy to reach 0 after 69px: 69 = v0^2/(2*0.28) => v0^2 = 38.6 => v0 = -6.2. So if vy0≈-6.2, apex 69px above launch. Why would vy0 be -6.2? sqrt(2*0.28*dist): if dist≈69 => v0=6.2. So dist≈69! That means `dist` is computed as ~69, i.e., `ty - this.y` ≈ -69?? No, this.y=808, ty=? If dist=69 then ty≈739. 

So targetY is ~739, not 181! Why? Because `H * rand(0.22,0.4)`... no that's 176-320.

Unless... `this.y = H + 8` = 808, and `ty` passed is actually `H*0.9`? No.

Wait, maybe the coordinates: the click at (500,300) -> launch(500,300). The demo launch(445,181). Both explode at 731. For click target 300, dist should be ~508, vy0~16.9, apex ~510px up -> y~298. For demo target 181, apex y~181. But both explode at 731. So targetY is being ignored/overwritten to ~739.

Where could targetY become 739? In Shell constructor: `this.targetY = ty;`. Then in update, `if (this.y <= this.targetY ...)`. 

Hmm unless `ty` is undefined! If `ty` is undefined, `this.targetY = undefined`, and `this.y <= undefined` is false, and then `this.vy >= 0` triggers at apex. That would give correct apex based on dist. But dist = hypot(tx-x, ty-y) with ty=undefined -> NaN -> vy NaN. Not that.

Wait, the explosion at 731 for both suggests apex ≈71px above 808, i.e., v0≈-6.3 for all. That matches dist≈69-71 for all. dist = hypot(tx - this.x, ty - this.y). If tx-this.x is small (jitter ~±W*0.05=±60) and ty - this.y = targetY - 808. For dist≈70, targetY-808 ≈ -70 => targetY≈738.

So targetY is ~738. The demo passes H*rand(0.22,0.4). Unless `H` is not 800 but something like... if H is 0? Then H*rand=0, targetY=0, dist large. Not.

Hold on, maybe I have the launch call wrong for the demo: `demo()` does `launch(W * rand(0.3, 0.7), H * rand(0.22, 0.4))`. But the click handler passes `e.clientY - r.top`. That's 300. Both yield ~738 target? No.

Actually, re-examine: EXPLODE logs show x drift: launch 445 -> explode 393 (-52). Launch 496 -> explode 448 (-48). vx = (tx - this.x)/(|vy|/0.28). tx-this.x is negative (jitter positive), vx negative small. Over ~4 frames, x changes by vx*4. If vy0≈-20 then |vy|/0.28≈74 frames, vx=(tx-x)/74. For x to move -52 over 4 frames, vx=-13, meaning (tx-x)=-962. That means this.x = tx+962, jitter huge. Not.

Alternatively if explosion happens after ~40 frames: vx=(tx-x)/74; x moves vx*40 = (tx-x)*0.54. If tx-x=+60 (this.x is tx-60? no rand is tx + rand(-40,40)). If this.x = tx + 40, tx-x=-40, x moves -21. Over 40 frames. Close-ish.

I'm going in circles. Let me just add a console.log inside explode with the shell's targetY and vy. Better: change explode call to pass debug info, or log in update right before explode. Let me log `this.y, this.targetY, this.vy, this.life`.

Actually simpler: I suspect the issue is the explode trigger fires at `life > 3.2`. If the shell's life increments beyond 3.2, and dt maybe huge? dt clamped to 3. life += 3 each frame -> after ~2 frames life>3.2! That's it!!!

Because dt is normalized to 60fps frames (dt=1 per 16.67ms). But `this.life += dt` treats dt as frames. Over 3.2 frames (53ms) the shell explodes due to `life > 3.2`. I intended seconds but dt is in frame units! So shells explode after ~3 frames, near launch. That's the bug! Same for Particle.maxLife=rand(1.1,2.4) treated as frames => particles live ~1-2 frames, nearly invisible. And `tr.life -= 0.006*dt` fine.

Yes! The dt normalization: dt = elapsed/16.667, so dt is in "frames" (~1). I used it as if seconds in life logic. I need life in seconds: either use dtFrames for physics and seconds for life, or make dt seconds and adjust constants.

Simplest fix: pass both or convert. Let me make dt represent seconds: dt = (now-last)/1000 clamped. Then physics constants need scaling: gravity was 0.28 px/frame². In px/s² that's 0.28*60² = 1008. Velocities were px/frame; speed 20 px/frame = 1200 px/s. This requires rewriting.

Cleaner: keep dt in frame units (call it `df`), and for life use seconds = df/60. Let me define `var df = ...; var dts = df/60;` and use dts for life timers, df for physics (gravity per frame²).

Trail timer: this.trailTimer += dt (frames) and threshold 0.6 — intended every 0.6s = 36 frames. Should use seconds. Let me use dts.

Particle.maxLife rand(1.1,2.4) seconds; life -= dts.
Trails life -= 0.006*dt should be 0.006*... hmm trails life go from 1 to 0 in 1/0.006=166 frames = 2.8s if dt frames. Actually 0.006 per frame *166. With df it's ~2.8s. Fine, keep df maybe. Let me use dts and 0.35 coefficient for ~2.8s. Simpler: keep `tr.life -= 0.006 * df`.

Let me restructure the frame loop with `df` (frames) and `dts` (seconds). And update Shell/Particle signatures to take both, or store module-level `DT_F`/`DT_S`.

Simplest: keep update(dt) where dt is frames for physics, and use a global `frameDtSec`. Let me just modify:
- frame computes `df` and `dts`.
- Shell.update(df, dts): life += dts; trailTimer += dts; physics with df.
- Particle.update(df, dts): physics df; life -= dts.
- trails life: `tr.life -= 0.35 * dts` (≈ full fade ~2.8s).

Also the explode triggers based on life>3.2 (seconds) correct. And `vy>=0` apex.

Also shell vy formula: gravity is 0.28 per frame². vel sqrt(2*0.28*dist) correct. Physics uses df (=1 at 60fps). Good.

Let me also reconsider `this.trailTimer > 0.6` now in seconds -> every 0.6s, fine.

Let me rewrite. Let me view the current file fully to apply edits.

Now I'll refactor to use `df` (frame units) for physics and `dts` (seconds) for lifetimes. I'll pass both to update methods. Let me apply edits.

Now the shell life > 3.2 is seconds and correct. Trail timer 0.06s = ~3.6 frames, good.

Let me also reconsider: gravity on ascent is 0.28 per frame². Shell vy0=-20 -> time to apex ~71 frames ~1.2s. Good.

Particle speed 1.2-6.4 px/frame -> up to 384px/s, spread radius up to ~300px over 2s. Good.

Now let me verify. Let me reload and take a motion capture. I'll keep the temp auto-launch and debug for now.

Now take a motion capture to see ascent and burst.

Excellent! Now it works beautifully. Motion frames show:
- Frame 1: a burst spreading (dim yellowish particles) plus descending.
- Frame 2: shell ascending with trail, bright explosion near apex.
- Frame 3: full radial burst spreading.
- Frame 4: particles falling under gravity, spreading outward and downward.

Console logs confirm: LAUNCH near target, EXPLODE near target (e.g., LAUNCH 394 192 -> EXPLODE 376 185), parts counts 60-99.

Now the visuals are grayscale-ish for some bursts — palette colors exist but maybe particle color alpha low. Actually frame3 burst looks white/gray. Some colored ones may exist. Fine.

Now I need to:
1. Remove the temp auto-launch interval.
2. Remove debug DOM element and on-canvas text and console.logs.
3. Verify click target accuracy and Clear.

Let me first verify clearance and click targeting before cleaning. Let me click at a specific location and check explode happens near it. Click at (300,200).

Actually console already showed launches with matching explode near target. Good.

Let me test Clear: click Clear button and confirm shells/parts/trails become 0. But auto-launch will re-add. Let me remove auto-launch first, then test Clear.

Let me clean up all debug now.

Let me read the file to confirm there are no leftover references (fcount, lastDt, dbg, sy2) and correct structure.

Good. Remove the "(no debug)" comment too. Minor. Let me remove it.

Now let me verify final: reload, wait for demo, motion to see demo, click test, clear test.

Since debug is removed, I'll verify via screenshots and console (no logs now). Let me reload and use motion to catch the demo. Demo fires at 500ms; motion starts some time after reload, so may catch it. Let me reload then immediately motion.

Actually let me remove the comment first.

No debug overlay. Now let me verify the demo and click with motion. Reload already happened; demo at 500ms likely already fired and gone by now. Let me do a fresh reload and immediately motion in the same message? They're dependent (motion after reload). Let me do reload then motion.

Actually I just reloaded. Let me take motion now; demo may still be active if within 2s. But some time passed. Let me just click and motion to verify interaction.

Click at (350, 250) then motion.

The shell launched from bottom (x≈348, y≈745) with a trail going up to around (395,281) faintly — ascending toward click target (350,250). Good, and the trail shows the path. Let me motion to see the burst.

Hmm, these motion frames only show the trail path (dark) and no bright burst. It seems the capture happened after the burst faded, or the burst particles are dim. Frame1 shows a fan/radial trail pattern centered near (350,150)... actually the fan suggests the burst happened at around (350,150) and particles fell, leaving trails? The persistent trail marks are left by the shell only during ascent, not particles. So the fan shape must be particles smearing. But they're very dark.

Wait, the fan shape is centered at top ~(350,150) and spreads downward. That's the shell trail going up plus... Actually the shell ascends fairly vertically. The fan appearance is odd.

The key concern: the burst doesn't appear bright. Earlier motion capture (during temp auto-launch) showed bright bursts. Now it seems dim. Why? Possibly because these frames are captured after particles faded. The trail path remains as faint marks.

Let me just take a screenshot right after a click and look. Actually the click screenshot showed the ascending shell. The burst takes ~1.2s. The motion tool first frame is immediately, then 0.55s. The click was already done in a separate call, and the shell may have been mid-flight. The motion frames then are 0.5-1.7s later — catching late particles.

The earlier successful capture had continuous launches, so bursts were always present. To verify a specific click burst, I need to click and capture within the same operation. Since I can't, I could temporarily re-add the auto-launch, but I already saw it works. Actually the earlier successful motion (during debug) clearly demonstrated ascent, burst, spread, fall. That was with the same physics after the fix. Good enough.

The final concern: is the current code the same as the one that produced good bursts? Yes, I only removed debug hooks and the auto-interval. The click behavior is intact.

But wait, in the current motion frames, no bright burst at all — just dark trails. Could it be that the burst now happens but particles render too dim? In the earlier good capture, bursts were bright white/yellow at the moment of explosion and spread. Here, the fan pattern around (350,150) is likely the burst, already faded. The center appears near y=150, which is above the click target y=250 — hmm, shell overshoots target by a bit due to extra rand(1,3) velocity, so burst around y=150-180. Fine.

Actually the persistent shell trail marks are visible as the vertical dotted line, and the fan is... particles leave no persistent trails. The fan must be motion-blurred particles. By frame1 of motion (taken ~1-2s after click), particles are in late fade. So dim. Makes sense.

I'm fairly confident. Let me do a final check on resize and Clear. For Clear I can click the Clear button and confirm the canvas is emptied. Since no debug, I'll inspect visually; but screenshot may show leftover faded pixels. Clear explicitly refills canvas and clears arrays, so it should be black. Let me click Clear and screenshot. The demo only fires on load, so after Clear nothing will re-appear. 

Let me click Clear, then screenshot.

Clear worked — scene is empty (black), and even the crease column artifact from before is gone. Hint reappeared? The hint text is not visible now (it was faded to opacity 0 after earlier). Actually clearAll sets hint opacity to 1, but hint may be off-screen or the screenshot doesn't show it. It doesn't matter.

Now let me verify resize keeps controls usable. I can't easily resize the viewport with the tool. But resize handler is registered and recomputes canvas dimensions and re-fills. The Clear button is fixed-position, so it stays. I've tested it doesn't break. I can trigger a resize by... the browser tool doesn't offer resizing. I'll note it as unverified.

Let me do one more verification: the demo shell on initial load actually fires. Let me reload and immediately motion (two calls). The demo fires at 500ms. Motion frames at 0, 550, 1100, 1650ms — should catch the demo ascent and burst. Let me reload then motion.

These frames again show the fan pattern but dim. The demo did fire (the fan center around 500,250 area). It's dim because captured later in the fade. Hmm, but frame 1 (at +0ms of motion) should be ~1-2s after reload; demo fires at 0.5s, ascent ~1.2s, so burst at ~1.7s. Frame1 may catch mid-ascent. Yet the image shows a fan already. The trail path is visible vertical from bottom to ~(500,270), and fan center around (500,300).

Wait, this fan pattern with radiating lines from a point is exactly a burst. It's dim because particles' alpha = t² and the shadow. Actually at burst moment particles should be bright opaque. Why so dim?

Let me reconsider: in the earlier auto-launch capture, bursts appeared bright white. Those were captured right at explosion. Here the capture timing lags.

The screenshots are captured with some delay after the "Frame N" timestamp. The images may be from a different instant than the frame label. Since the motion tool likely composites after capture, and the browser is running in real time, by the time the screenshot is encoded the scene may have advanced.

Regardless, I've seen the burst/particles render brightly and correctly. The core functionality is verified.

One concern: the persistent trail marks remain visible as faint vertical lines and the "fan" artifact seems to persist across all four frames unchanged, suggesting it's NOT live particles but stale canvas pixels that never fully clear! The motion-blur fill uses `rgba(5,6,15,0.28)` each frame. After many frames, residual should fade to near zero. But if frame rate is low, could persist. Over 1.7s at 60fps = 100 frames, 0.72^100 ≈ 0. So it should be gone. But the fan looks identical in all 4 frames — suspicious. It could be that the canvas isn't being cleared because... hmm.

Actually, the fan is very dark (like #0a0c18 vs #05060f). It may be the trails (persistent marks) which decay slowly (0.35/sec => 2.8s). The fan could be trails pushed by `explode` (6 marks at center) plus shell trail. But a fan of ~20 lines? explode only adds 6. Hmm.

Wait — actually the fan pattern may be the motion-blur accumulation of particles that were bright earlier, leaving afterimages on the canvas that fade slowly due to the 0.28 alpha (72% fade per frame isn't instant; at 60fps it's fast though).

I think these are just faint residual afterimages and it's fine. The very dark appearance suggests the capture is at a late stage.

Let me do one definitive check: reload and take a plain screenshot very quickly? The screenshot call itself is fast but the demo is at 500ms. Let me instead click and immediately screenshot (click returns fast). Earlier click screenshot showed the shell mid-ascent clearly. That verified launch targeting.

I'm confident the implementation meets requirements. Let me do a final code review for correctness and any leftover issues, especially the resize handler and Clear.

Let me read the full final file.

The code looks good. One issue: `window.addEventListener("load", ...)` — if the script runs and the load event already fired (e.g., cached), demo won't fire. Inline script at end of body always runs before load, so fine. But to be safe, I could just call demo via setTimeout directly instead of waiting for load. Actually there might be a subtle issue: the iframe sandbox loads the page; load fires after. Fine.

Another concern: resize re-fills the canvas but then the main loop continues with motion blur; controls stay. Good. But resizing clears trails — acceptable.

One improvement: clearAll sets hint opacity to 1, which reappears after Clear. That's fine/useful.

Potential issue: The `resize` function fills the canvas, but the loop also fills each frame. Good.

The `Shell` overshoots target noticeably because vy adds extra rand(1,3); with vy ~-20, extra 1-3 gives apex higher by ~ (extra velocity adds height). Actually apex difference: v0 larger by 2 => height increase ≈ (v0²-v0'²)/(2g)... it's small. Fine.

Everything required is present:
- click launches visible shell from bottom toward click ✓
- explodes into radial burst ✓
- gravity, fade, removal ✓ (life in seconds, dead removal)
- several overlapping shells and bursts supported ✓ (arrays)
- demo shell on load ✓
- Clear removes shells/particles/trails ✓
- controls usable after resize ✓ (fixed UI, resize handler)
- procedural drawing, no external resources ✓

I verified via browser: initial demo, click targeting, burst, spread/fall, Clear. I should confirm no external resources: no external links. `backdrop-filter` is CSS, fine.

Let me do a final quick check that there are no console errors after the cleanup (reload returned only the sandbox warning). Good.

I think we're done. Let me write a brief report. Also note the sandbox warning is from the harness, not our code.

One last thing: the demo might not be super visible because it fires at 500ms and by the time a user looks... it's fine, it bursts around 1.7s.

Let me make one small robustness improvement: call demo regardless of load event by using a direct check. Actually, simpler: replace load listener with an immediate `setTimeout(demo, 500)` and the hint timeout. That guarantees demo. Let me do it to be safe.

Loads cleanly, no errors. Let me do one final motion capture to confirm demo fires now (should appear within ~0.5-1.7s). Since reload + motion are separate calls, there's a gap; the demo may already be gone. But earlier I confirmed demo and click work. Let me do a final motion right now to see if anything remains. Actually let me just verify one burst via click to be thorough with the final code, then report.

Click at (850, 300) and screenshot immediately, then motion.

The click at (850,300) launched a shell at x≈810, y≈760 (bottom), with a trail. There's also a faint artifact upper-left (demo burst residual) and a vertical dim line at x≈425 (demo trail). Good.

Let me do a final motion to capture the burst from this click.

These frames again show only faint residual afterimages (the radial fan pattern and vertical trail) that don't change much. This is concerning: the scene seems "stuck" with these faint patterns. They appear to never fade away because the motion-blur fill alpha 0.28 leaves ~0.72^n, and at 60fps over 1.5s that's essentially zero. Unless the frame rate is very low during capture, but even 30 frames -> 0.72^30 ≈ 5e-5. So these patterns should vanish quickly.

But they persist identically across all 4 frames. That means the canvas is NOT being updated between these captured frames, OR the frame loop stopped.

Wait — maybe rAF stops when there's no interaction, and the "motion" tool only captures the initial frame and re-renders? But earlier with the debug, frame counts increased during motion (f=1015 -> 1045 -> 1081 -> 1111), so rAF WAS running during motion captures. And the burst was clearly visible.

Now, with no debug, the frames show stale patterns. Maybe the particles are actually there but very dim, and the fan is the current live particles (all falling). But they'd change position across frames. The fan looks static.

Actually, the fan pattern is centered around (830,300) — the click target. It looks like a frozen burst. And the vertical line at x≈425 is the previous demo trail. These static patterns could be... wait, could the canvas have `image-rendering` issue? No.

Let me think: the motion tool captures 4 frames from a "motion" recording — maybe it records the animation over 1.7s and returns 4 sample frames. The browser runs continuously. The fact that frame counts increased earlier proves it. So the scene should be live.

Maybe the issue is that during `motion`, the tool pauses the page and captures, then resumes. The patterns we see are the state at capture. They look similar because the burst evolves slowly at late stage. The vertical trail at x=425 is persistent trail marks from the demo (life decays 0.35/s, so ~3s). The fan around 830 might be persistent trail marks too? No, particles don't create persistent trails. Only shells do. So the fan must be live particles.

Hmm, but a fan of thin lines is not what live particles look like; they'd be dots/spread. The thin-line fan is motion-blur streaks from bright particles moving fast — captured over exposure? The screenshot captures a single frame, no motion blur except the canvas's own accumulation.

Actually the canvas accumulation IS the motion blur: each frame draws semi-transparent dark overlay, so a bright particle moving leaves a fading streak. At a fast capture, particles leave streaks. That's the intended "trails" effect. So the fan is the trail of the burst particles. 

Why dim? Because capture is at late fade. The earlier bright capture was at the burst moment.

I'm now fairly confident the dimness is a timing artifact of the harness, and in a real browser the effect is bright and dynamic. But let me confirm by capturing a burst more reliably. I could temporarily slow down particle fade so I can catch brightness... but I already saw it bright earlier. Let me not over-engineer.

Actually, wait. Let me reconsider whether the particle brightness is adequate. In Particle.draw, alpha = t² where t = life/maxLife. At burst t=1, alpha=1, shadowBlur=10, size ~2.8. Bright. Over 2s fades. Good.

The earlier post-fix motion captured a brilliant white burst and colored particles. That's conclusive.

Final answer: implementation complete. Report what was verified.

Let me make sure the final file is saved (it is). Also verify no external URLs.

Everything checks out. Let me write the final report.

Actually, let me reconsider whether I should verify resize. The requirement "Keep controls usable after resizing." The Clear button is position:fixed so it stays. The resize handler reconfigures canvas. I can't resize the viewport in this harness. I'll note it unverified but explain the mechanism.

Let me also double check clearAll after resize: it fills W,H which are updated. Good.

Final report.