fix: stabilize messages timeline virtualization

[juliusmarminge]

Summary

• Extracted timeline row derivation and row-height estimation into shared logic so the virtualizer uses the same row model as the renderer.
• Improved changed-files height estimation by accounting for nested tree content, and surfaced virtual row metadata for debugging and verification.
• Fixed ChangedFilesTree to respect the initial collapse state on first render and preserve expansion state across updates.
• Added regression coverage for the collapsed directory rendering path and a browser harness that checks virtualization estimates against rendered heights.

Testing

• Not run (PR description only).
• Added ChangedFilesTree unit coverage for the initial collapse-state behavior.
• Added Vitest browser coverage for timeline virtualization estimate mismatches and changed-files collapse/virtualizer synchronization.

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Note

Medium Risk
Touches chat timeline virtualization and row-height estimation; mistakes could cause scroll-jank, incorrect cached heights, or missing content during expand/collapse, though changes are UI-only with no auth/data risk.

Overview
Stabilizes chat timeline virtualization by extracting a shared row model + height estimator (deriveMessagesTimelineRows, estimateMessagesTimelineRowHeight) and using it for virtualizer sizing, including work-group expansion state and changed-files card height.

Prevents stale virtualizer measurements on resize by scoping react-virtual item keys to the current timeline width, and adds optional onVirtualizerSnapshot plus extra data-* attributes to aid debugging/verification.

Fixes ChangedFilesTree initial expand/collapse behavior by replacing effect-driven directory expansion state with an override map keyed by expand-mode + directory set, and adds unit + browser harness tests to catch estimate/measurement drift and ensure collapse/expand keeps the virtualizer in sync.

^{Written by Cursor Bugbot for commit 32ef605. This will update automatically on new commits. Configure here.}

Note

Stabilize messages timeline virtualization with improved row height estimation

• Extracts timeline row derivation and height estimation into MessagesTimeline.logic.ts, grouping contiguous work log entries and handling proposed-plan, message, and working row kinds
• Scopes virtualizer item keys by rounded timeline width to prevent stale measurement reuse on resize
• Improves estimateMessagesTimelineRowHeight to account for changed-files cards, work group expansion/overflow, proposed-plan markdown length, and distinct assistant vs. user line heights
• Reworks ChangedFilesTree expansion state to use a keyed { key, overrides } object instead of useEffect sync, so directories default to expanded/collapsed based solely on allDirectoriesExpanded
• Adds an optional onVirtualizerSnapshot prop to MessagesTimeline for observing virtualizer measurements, plus a browser-based test suite validating height estimates against real renders

^{Macroscope summarized 32ef605.}

[macroscopeapp]

Approvability

Verdict: Approved

This is a self-contained bug fix for timeline virtualization, extracting height estimation logic, improving accuracy of estimates, and adding proper cache invalidation on width changes. The changes include comprehensive browser tests and are limited to UI behavior without touching sensitive areas.

^{You can customize Macroscope's approvability policy. Learn more.}

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