fix: replace AlignmentBarrierLayout with _FrameLayout to stop alignment cascade and message disappearing

[devin-ai-integration]

Summary

Removes AlignmentBarrierLayout — a custom Layout container that was wrapping the ScrollView to block alignment query cascades. While it successfully prevented the O(n) explicitAlignment hang, it disrupted SwiftUI's internal scroll infrastructure, causing messages to disappear on send, resize, scroll, and conversation switch (intermittently at first, then constantly after the LUM-811 streaming cache change increased body re-evaluation frequency).

Root cause of the hang was _FlexFrameLayout, not alignment queries reaching the LazyVStack directly. The fix replaces the two .frame(maxWidth:) modifiers (which create _FlexFrameLayout) with a single .frame(width:) (which creates _FrameLayout). _FrameLayout returns bounds.midX for alignment without querying children — the cascade stops here without needing any custom Layout in the view hierarchy.

Changes

1. MessageListView.swift — Remove AlignmentBarrierLayout wrapper; unwrap ScrollView and all scroll modifiers back to the outer chain. Replace .frame(maxWidth: chatColumnMaxWidth).frame(maxWidth: .infinity) with .frame(width: containerWidth > 0 ? min(containerWidth, chatColumnMaxWidth) : chatColumnMaxWidth).
2. AlignmentBarrierLayout.swift — Deleted (no other usages in the codebase).
3. AGENTS.md — Replace barrier documentation with _FrameLayout vs _FlexFrameLayout guidance. Explicitly warns against custom Layout barriers near ScrollView.

Resulting modifier chain

ScrollView { scrollViewContent .background(ScrollObserver) }
  .scrollContentBackground(.hidden)
  .scrollDisabled(...)
  .defaultScrollAnchor(.bottom, for: .initialOffset)
  .scrollPosition($scrollPosition)
  .environment(\.suppressAutoScroll, ...)
  .onScrollPhaseChange { ... }
  .scrollIndicators(...)
  .id(conversationId)
  .frame(width: containerWidth > 0 ? min(containerWidth, chatColumnMaxWidth) : chatColumnMaxWidth)
  .overlay(alignment: .bottom) { ScrollToLatestOverlayView }
  .onAppear { handleAppear() }
  ... [remaining lifecycle modifiers]

No AlignmentBarrierLayout. No .frame(maxWidth:) on this path.

Review & Testing Checklist for Human

CI skips macOS builds — all items require local Xcode verification. Clean build recommended (Cmd+Shift+K, delete DerivedData).

• Messages no longer disappear: Open a long conversation. Send a message, resize the window, switch conversations, scroll up and back down. Messages must remain visible throughout. This was the primary regression from the barrier approach.
• Alignment hang is still fixed: Open a conversation with many tool calls. Resize the window repeatedly. Spindump should show near-zero explicitAlignment hits in the LazyVStack subtree. The _FrameLayout blocking the cascade is based on SwiftUI internals analysis (Apple doesn't document this explicitly) — the spindump is the definitive check.
• Width, centering, and scroll area: The old .frame(maxWidth: 808).frame(maxWidth: .infinity) made the scroll area fill the full window width (scrollbar at window edge) with content centered at 808pt. The new .frame(width: computed) makes the scroll area exactly min(containerWidth, 808) wide. Verify: (a) chat column is centered and capped at 808pt on wide windows, (b) scrollbar position is acceptable, (c) fills correctly on narrow windows.
• First-frame flash on narrow windows: With sidebar open (narrow chat area), reload the app. Check if there's a visible one-frame width jump as containerWidth goes from 0 → actual. The fallback is 808pt (MainWindowView.minWidth is 800, so this should be safe).
• Thinking indicator dots: Verify the three-dot animation still works correctly within its bubble.

Notes

• The 23 cell-level HStack+Spacer conversions from LUM-800 remain as defense-in-depth — they prevent any future .frame(maxWidth:) added above from cascading into cells.
• The .overlay(alignment: .bottom) for ScrollToLatestOverlayView is safe: it applies to the result of .frame(width:), and _FrameLayout with a definite width does not query children for alignment.
• The PodRuntimeTests CI failure (AppleContainersPodRuntime macOS 26.0 availability) is pre-existing and unrelated.

Link to Devin session: https://app.devin.ai/sessions/ef66aadc67724413b3b5ede76cf20ca9
Requested by: @ashleeradka

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✅ Devin Review: No Issues Found

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

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