LUM-799: Replace GeometryReader with .onGeometryChange(for:) in ChatView

[devin-ai-integration]

Summary

Replaces the GeometryReader wrapper in ChatView.body with a @State property + .onGeometryChange(for:) modifier to eliminate 48–83 second performance hangs.

GeometryReader as a layout container creates a GeometryReaderLayout node whose child measurement runs during placeSubviews. Any attribute graph invalidation (hover, window focus, animation tick) forces a full O(n × depth) re-measurement cascade through the LazyVStack. .onGeometryChange is a post-layout observer — it reads geometry after layout completes without participating in the measurement chain, and its action only fires when the extracted value actually changes (Apple docs).

The downstream data flow is unchanged: containerWidth still flows through mainContentStack → activeConversationContent → MessageListView → bubbleMaxWidth environment key.

Because @State containerWidth initializes to 0 (unlike GeometryReader which provided the real width from the first render), the first .onGeometryChange callback produces a 0→real transition. Without mitigation, this would trigger spurious scroll resize stabilization during initial load. A guard in handleContainerWidthChanged() now absorbs this first measurement (when lastHandledContainerWidth == 0) by recording the width without starting stabilization. Subsequent real resizes still trigger normally.

Review & Testing Checklist for Human

• Build in Xcode — CI skips all macOS checks; this must be verified locally
• Test window resizing — resize the window (including very narrow < 808px) and verify chat bubbles reflow correctly, scroll position is maintained, and no hangs occur
• Hover/focus stress test — hover over messages, switch window focus back and forth rapidly, and confirm the UI stays responsive (this was the original hang trigger)
• Verify initial scroll position on app launch — open a long conversation from cold start and confirm it scrolls to bottom without a stutter. The new lastHandledContainerWidth > 0 guard skips stabilization on first mount, so verify this doesn't regress the initial-load scroll-to-bottom behavior
• Conversation switching — switch between conversations and verify scroll position resets correctly. handleConversationSwitched() seeds lastHandledContainerWidth = containerWidth — confirm this still works when the width was previously recorded

Recommended test plan: Open a long conversation, resize the window several times (including very narrow), hover over messages, switch window focus, switch conversations, and verify no hangs or scroll position issues.

Notes

• .onGeometryChange(for:of:action:) requires macOS 14+; deployment target is macOS 15.0 — safe
• This reverts the approach from commit bf7d13875 (PR refactor: ChatView uses GeometryReader instead of @State for container width #23855) which introduced the GeometryReader, contradicting AGENTS.md guidance. The @State write concern that motivated that commit is moot because .onGeometryChange deduplicates via Equatable
• Already used in ~12 other places in the codebase
• For narrow windows (< 808px), the first frame renders bubbles at the 760px default before .onGeometryChange fires with the real width — verify whether this causes a visible layout jump

Link to Devin session: https://app.devin.ai/sessions/14a69bbf31b145c987e81d2aee0c2612
Requested by: @ashleeradka

---

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[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 1 additional finding.

[ashleeradka]

✦ Reviewed ChatView.swift in full context.

GeometryReader removed, replaced with @State containerWidth + .onGeometryChange(for: CGFloat.self). containerWidth=0 initial state handled gracefully — existing ternary in MessageListContentView falls back to VSpacing.chatBubbleMaxWidth until first callback. Same behavior as before.

Clean single-file change, no conflicts with main.

Ship it.