Every pixel oracle tried for STA-2694 was blind: `drawImage` on a
non-preserveDrawingBuffer WebGL canvas returns a re-rendered copy, and
Playwright's screenshot drives a fresh compositor frame that heals a stale paint
before capture. Reading pixels is self-defeating here — the read triggers the
repaint that hides the bug.
Count the WebGL draw commands instead, by wrapping GlyphRenderer.updateCell and
gl.drawElementsInstanced on the live pane. A draw command cannot be healed after
the fact, so "did the reveal actually repaint?" becomes directly observable.
Teeth-verified: removing releaseAbandonedSynchronizedOutput from
schedulePaneRevealPresent fails the stranded-latch test.
Two findings, both of which change previously-committed claims:
1. The 1s watchdog does NOT bound the synchronized-output defect. It is armed
only inside `bufferRows`, and `refreshRows` returns at its `_isPaused` check
first — so while a pane is occluded nothing reaches `bufferRows` and no timer
is ever pending. A pane hidden mid-`?2026h` holds the latch with no watchdog
behind it, indefinitely. ed1eaf55f1's "closes a bounded window" scope note was
wrong; this is the unbounded garble the report describes, and the fix closes
it. Corrected in the module doc comment.
2. It refutes the diff-based-staleness hypothesis behind 0f7ec4458d (reverted in
8d5eacecb4). `_updateModel` does early-continue per unchanged cell, but
`GlyphRenderer.render` then copies vertices for EVERY row up to
`lineLengths[y]` and issues ONE full-viewport draw — measured identical
instance counts (562) for a diff-skipped and a model-cleared refresh, with
updateCell at 0 vs 561. The DOM renderer likewise replaceChildren()s every
row unconditionally. Clearing the model could not change what reached the
screen, and `_clearModel(true)` zeroes every glyph vertex while
`RenderService.clear()` fires no repaint of its own — so it opened a
blank-viewport window (also asserted here) for no benefit.
Also keeps the idle-agent and headful desktop-hide cases from the reverted
commit, since those were independent of the refuted production change, and
rewrites the alt-screen spec header to point paint questions at this oracle.
Refs STA-2694.
Co-authored-by: Orca <help@stably.ai>
NeilandOrca
3ccffc17ec