test(xterm): A/B observer-nullify variant vs WeakRef (upstream #5821)

[srid]

Purpose

Maintainers on xtermjs/xterm.js#5821 (jerch, Tyriar) pushed back on the WeakRef wrap and suggested storing the IntersectionObserver on this and explicitly nulling it on dispose — without WeakRef. Per W3C spec, disconnect() does not release the callback (the observer retains it so .observe() can be re-armed), so their hypothesis is: the observer itself is the JS-reachable root, and dropping that ref lets the whole chain GC.

This draft PR points the xterm override at a stacked test branch that implements that variant, so we can run a prod Task Manager A/B on pureintent.

Variant

juspay/xterm.js@test/kolu-observer-nullify-built stacks one commit on top of the existing fix/kolu-xterm-fixes-built. The diff (src/browser/services/RenderService.ts):

private _observer: IntersectionObserver | undefined;
...
this._observer = new w.IntersectionObserver(
  e => this._handleIntersectionChange(e[e.length - 1]),
  { threshold: 0 }
);
this._observer.observe(screenElement);
this._observerDisposable.value = toDisposable(() => {
  this._observer?.disconnect();
  this._observer = undefined;
});

No WeakRef. Strong this capture in the callback; observer ref hoisted onto the instance and nulled on dispose.

Local test was inconclusive

Ran a 30-toggle canvas-maximize A/B under chrome-devtools MCP (headless). All three variants — unpatched, WeakRef, observer-nullify — showed identical benign heap diffs: no JSArrayBufferData / Uint32Array / BufferLine growth. The MCP Chrome lacks whatever instrumentation/extension caused the original retention in real-world Chrome, so local can't distinguish the variants. Prod Task Manager on pureintent is the actual arbiter.

Repro recipe (updated for mode-less workspace)

The original #617 repro was Focus ↔ Canvas mode toggle, which no longer exists (#622 made the workspace mode-less — the canvas is always on). The modern equivalent that still cycles Terminal mount/unmount is the canvas-maximize toggle — the Maximize button on each tile's chrome.

Why it works: toggling canvasMaximized flips TerminalCanvas.tsx:352-370 between <For each={tileIds}> and <Show when={maximized && activeId} keyed>. With 7 tiles, one toggle cycle produces 8 Terminal disposes + 8 Terminal mounts (7 tiled tiles → 1 maximized, then back). Over 30 cycles that's 240 RenderService create/dispose pairs — the same surface as the original repro. Confirmed locally via window.__kolu.lifecycle(): mounts increments from 15 → 255, cleanups from 8 → 248 across 30 cycles, with 7 live terminals throughout.

Steps

1. Fresh tab on pureintent with 7 terminals restored (Task Manager: capture baseline Memory Footprint)
2. Paste in DevTools console:

   (async () => {
     for (let i = 0; i < 30; i++) {
       document.querySelector('[data-testid=canvas-tile-maximize]').click();
       await new Promise(r => setTimeout(r, 250));
       document.querySelector('[data-testid=canvas-tile-maximize]').click();
       await new Promise(r => setTimeout(r, 250));
     }
   })();

3. Capture post Memory Footprint
4. Compute Δ / 30 toggles

Interpreting the Δ

• ≈ 0 MB → observer-nullify works. Reply to #5821 with prod evidence and simplify the upstream PR.
• ≈ +367 MB (pre-fix(xterm): patch IntersectionObserver retention (-367 MB/30 mode-toggles) #617 number) → observer-nullify doesn't defuse the retention. Reply to #5821 with that evidence and defend WeakRef.
• Anything in between — note the magnitude, still lands as data on #5821.

Fallback if canvas-maximize doesn't stress the path

If the prod Δ with canvas-maximize is implausibly flat (possible — WebGL addon lifecycle could mask the IntersectionObserver path through some unrelated interaction), fall back to the close/recreate cycle: 30× Ctrl+Enter followed by Ctrl+D per terminal. That exercises the exact same RenderService lifecycle at 30× the rate of the maximize toggle, and was already a documented repro in the perf-diagnose skill.

Rollback

Not for merge either way. Close draft after data is collected; the actual decision lands as a change to the upstream PR #5821 (either simplify to observer-nullify or keep WeakRef).

Refs xtermjs/xterm.js#5821, #617, #620