fix(macos): coalesce ComposerTextEditor.measureHeight and skip redundant ensureLayout passes (LUM-827)

[devin-ai-integration]

Routes every measureHeight call site in the chat composer through a DispatchWorkItem-based scheduler and adds a (textStorage.hash, textContainer.containerSize.width) skip-guard inside measureHeight, so upstream SwiftUI state cascades, scroll/resize notifications, and per-keystroke textDidChange no longer stack O(n) synchronous NSLayoutManager.ensureLayout passes that were blocking the main thread for 2s+ on large composer content (LUM-827, Sentry VELLUM-ASSISTANT-MACOS-EH). All layout work stays on the main thread (NSLayoutManager is not thread-safe), the TextKit 1 stack and IntrinsicScrollView.contentHeight one-way flow are untouched, ensureLayout is still invoked whenever inputs change so usedRect never returns stale values, and dismantleNSView cancels any pending work.

What changed

• Coordinator.scheduleMeasureHeight(_:) cancels any pending DispatchWorkItem and enqueues a new one via DispatchQueue.main.async — rapid bursts collapse into one ensureLayout pass on the next main-queue turn.
• measureHeight early-returns when (textStorage.hash, containerSize.width) match the last measurement. NSAttributedString.hash is content-and-attribute based (Foundation), so it invalidates on any edit; containerSize.width (under widthTracksTextView = true) captures window/split-view resize.
• invalidateMeasureHeightCache() is called in the fontChanged || colorChanged branch of updateNSView as a belt-and-braces fallback for typography mutations that may not produce a deterministic hash delta.
• All five call sites (frame observer, bounds observer, makeNSView initial measurement, textDidChange, and the textWasExternallyReplaced || fontChanged-guarded re-measure in updateNSView) go through scheduleMeasureHeight so they coalesce across a single event.
• dismantleNSView calls cancelPendingMeasureHeight() — no deferred layout pass fires against a torn-down view.

Builds on top of #25597 / LUM-832, which already converted the focus/cursor bindings to callbacks and guarded updateNSView behind change checks. That PR reduced the frequency of measureHeight calls; this one makes each call cheap when nothing layout-relevant changed.

Reviewer focus (highest risk first)

• Skip-guard correctness. Relies on NSAttributedString.hash changing whenever content or attributes do. The typography-change fallback is defensive; worth sanity-checking that font/theme changes actually re-flow the composer.
• One-runloop-tick defer. Every measurement (including the guarded updateNSView re-measure after external text replacement) now lands one DispatchQueue.main.async hop later. IntrinsicScrollView.contentHeight already smooths < 0.5pt deltas, so this should land within the same render frame — confirm no visible pop on mount, send (text → ""), or first paste of large content.
• Lifecycle. DispatchWorkItem captures [weak self, weak textView]; dismantleNSView cancels pending work. No retain cycle and no layout against a torn-down view.
• CI does not build macOS. Requires a local Xcode build to confirm it compiles. Behaviour to exercise manually: type, large paste, window resize, split-view resize, theme/font change, sending (text → "").

Alternatives considered and rejected

Documented so future work doesn't revisit dead ends:

• Moving ensureLayout off-main — NSLayoutManager is not thread-safe.
• Removing ensureLayout entirely — stale usedRect causes composer jump/clip (regresses LUM-622).
• Bounded ensureLayout(forGlyphRange:in:) — doesn't yield total content height.
• Dropping measureHeight for isVerticallyResizable + frameDidChangeNotification only — NSTextView resizes after layout, but SwiftUI needs the height at the start of the render cycle.
• TextKit 2 migration — out of scope; the explicit TK1 stack at L98–112 avoids a known TK2→TK1 downgrade bug.
• Storage-hash-only cache — misses window/split-view resize.
• Length-only invalidation — misses same-length glyph-width edits (e.g. font substitution).
• Timed debounce — introduces perceptible latency; next-runloop-tick is sufficient.

Related context

• #24557 / LUM-746 added Equatable on ComposerView/ComposerSection to reduce the frequency of updateNSView; complementary to this PR.
• #23672 established the IntrinsicScrollView.contentHeight one-way flow — preserved here.
• #24401 → reverted in #24417 bridge refactor is deliberately not reintroduced.

Similar ensureLayout patterns in HighlightedTextView.swift, VCodeView.swift, and SelectableTextView.swift are out of scope here and candidates for a follow-up with the same coalescing shape.

Test plan

• Pre-push hook (lint / type-check / design tokens): passing.
• CI: Socket Security checks passing; macOS Build / macOS Tests skipped by CI (macOS toolchain not available in CI).
• Local Xcode build + manual exercise of composer height tracking on type, large paste, window resize, split-view resize, theme/font change, and send (text → "") — pending by reviewer on macOS.

Link to Devin session: https://app.devin.ai/sessions/823de84dc8854081aea1f926200f39e5
Requested by: @ashleeradka

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.

[vex-assistant-bot]

✅ Approved — Clean coalescing fix for composer main-thread hang

LUM-827 — 2s+ hang from uncoalesced NSLayoutManager.ensureLayout in ComposerTextEditor.measureHeight.

Approach validation

• No circular fixes — git history shows no add/revert cycles on measurement coalescing
• No competing in-flight work — PR #25597 (cursor flicker) is complementary, not conflicting
• PR #24401 (bridge refactor) was reverted in #24417 — PR description correctly notes it's not reintroduced
• Sentry evidence (VELLUM-ASSISTANT-MACOS-EH) confirms the hang

What this does

1. DispatchWorkItem coalescing — All 5 measureHeight call sites now go through scheduleMeasureHeight, which cancels any pending work item before scheduling a new one via DispatchQueue.main.async. Rapid bursts (upstream SwiftUI cascades, frame+bounds notifications, per-keystroke textDidChange) collapse to a single ensureLayout pass on the next runloop turn.

2. Skip-guard — measureHeight now early-returns when (textStorage.hash, containerSize.width) haven't changed. This makes the unconditional scheduleMeasureHeight at the end of updateNSView safe — previously guarded by if textWasExternallyReplaced || fontChanged, the skip-guard is a more comprehensive check that catches ALL no-op cases.

3. invalidateMeasureHeightCache() — Called in the fontChanged || colorChanged branch of updateNSView as a belt-and-braces fallback for typography mutations that may not alter NSAttributedString.hash deterministically.

4. Lifecycle cleanup — dismantleNSView calls cancelPendingMeasureHeight() so deferred work never fires against a torn-down view.

Review notes

• Unconditional scheduleMeasureHeight in updateNSView replaces the old if textWasExternallyReplaced || fontChanged guard. This is correct — the skip-guard inside measureHeight handles the no-op case more comprehensively (hash+width vs. just text/font change detection). The deferred call is cheap when skipped (one hash comparison).
• DispatchWorkItem cancellation means during high-frequency streaming re-evals, only the last scheduled measurement fires. No accumulation of queued work items.
• NSAttributedString.hash is content-and-attribute based per Foundation — reliable for detecting text changes. Width check catches resize without text change.
• TextKit 1 stays on main thread — ensureLayout is deferred but never moved off-main. Correct per Apple docs for NSLayoutManager.
• IntrinsicScrollView.contentHeight 0.5pt delta smoothing is preserved — deferred measurement should land within the same render frame.

QA focus

• Composer height tracking on: type, paste, resize, split-view, window resize, font/theme change
• No visible height pop on mount or send (text → "")
• First paste of large content should not hang

Follow-up candidates (noted in PR)

• HighlightedTextView.swift:320 — same uncoalesced ensureLayout pattern
• VCodeView.swift:336 — same pattern
• SelectableTextView.swift:107,186 — same pattern