Four tightly-scoped hot-path fixes covering the highest-impact items in the
reporter's CPU profile (13.1 s profiled / 30 s window):
1. `event-controller.ts:handleEvent` no longer fires a blanket
`statusLine.invalidate() + ui.requestRender()` before every session event.
The pre-render was a leftover from #4145 when `updateEditorTopBorder()`
still eagerly rebuilt the border; the lazy provider added in #4145 made
it redundant. It fired on every `message_update`/`tool_execution_update`
during streaming — the pre-render's frame ran while the handler was
awaiting, then the handler's own `requestRender` scheduled a second
identical frame. Every handler that mutates visible state already calls
`requestRender()`.
2. `shimmer.ts:shimmerSegments` iterates the segment string in place instead
of building a code-point array with `Array.from(seg.text)` every animation
frame. Runs of same-tier chars are emitted via a single `slice` per run
rather than accumulating into `runBuf`. Surrogate pairs stay atomic — the
code-point index still advances by 1 per emoji. Microbench over 30k
frames: 45 ms → 18 ms (2.53x), allocation rate down from ~N-per-frame to
a handful per frame. New tests cover mixed BMP+surrogate and all-emoji
inputs. `Array.from` was the #1 self-time hotspot in the reporter's
profile at 10.2%.
3. `Markdown.setText` gains an equality guard mirroring `Text.setText`
(returns `false` when `text === #text`). Providers re-emit identical text
on ticks with no delta (throttled frames, reconciled tool-execution
updates); each of those now short-circuits instead of dropping
`#cachedLines` and forcing a full lex + wrap on the accumulated paragraph
(the reporter's #3 hotspot at 8.4%). New test asserts render-reference
stability + return-value semantics.
4. `SPINNER_RENDER_INTERVAL_MS` aligned with `SPINNER_GLYPH_ADVANCE_MS`
(both 80 ms). The previous 33 ms cadence emitted ~2.4 paints per glyph
step; the differential-output dedup only skips the write, not the
compose walk. Visually identical (glyph advance was already 12.5fps),
halves paints during tool execution.
Skipped (out of scope for a bug fix, deserve dedicated PRs):
- Freezing streaming prefix on single `\n` boundaries — correctness-bound
to `\n\n` block separators (CommonMark loose-list continuation).
- Compose-phase idle gate + adaptive-backpressure moving-average — need a
component-level dirty flag; the 200 ms cap in `#scheduleRender` was set
for a reason (#4145 tail-latency guard).
Tests updated: two IRC-expiry tests in event-controller-message-start.test.ts
that were asserting the pre-render's second `requestRender` call now expect
one.
Fixes#4353
roboomp
30527aee0c