fix(app): stabilize virtual session timeline interactions

[Hona]

Summary

• Preserve manual tool expand/collapse state when active assistant content streams and changes virtual row shape.
• Add a small virtua@0.49.1 Solid patch exposing VirtualizerHandle.measure() for synchronous mounted-item remeasurement.
• Use that measure hook for recent context group expansion so following rows reflow without visible overlap.
• Keep non-virtualized timeline diffs mounted across sibling/count-only updates; disable nested Pierre virtualization inside the already virtualized timeline.
• Parse patch-backed diffs through Pierre as partial metadata, including legacy headerless hunks, while preserving complete before/after inputs as complete Pierre diffs.
• Respect pinned Pierre beta lifecycle constraints: retained plain timeline diffs update in place, while virtualized viewers reset on changed target or construction-bound inputs.

Data-driven repros fixed

• A manually collapsed edit tool reopened when later assistant content streamed.
• Expanding a recent context group overlapped the following row before virtualizer measurement settled.
• A sparse multi-hunk patch could render omitted context as repeated undefined text.
• Headerless stored patches could render as empty diffs after direct Pierre parsing.

Scope

• Normal completed edit, apply_patch, snapshot, and VCS diffs are patch-backed and use the direct Pierre parsing path.
• Complete before/after payloads remain compatibility input and preserve Pierre's isPartial: false invariant; this PR does not optimize pathological patchless full rewrites.
• Upgrading @pierre/diffs and evaluating its new CodeView API for the dedicated review surface are separate follow-ups.

Tests

• bun test src from packages/ui
• bun typecheck from packages/ui
• bun typecheck from packages/app
• bun test --preload ./happydom.ts src/utils/diffs.test.ts from packages/app
• bun run test:e2e e2e/regression/session-timeline-collapse-state.spec.ts e2e/regression/session-timeline-context-resize.spec.ts e2e/smoke/session-timeline.spec.ts from packages/app
• bun turbo typecheck

Follow-up

• OpenCode currently pins @pierre/diffs@1.1.0-beta.18; stable 1.2.x adds CodeView and improved virtualized lifecycle/layout handling. Upgrade separately once scoped and eligible under the dependency release-age policy.

[Copilot]

Pull request overview

This PR stabilizes the session message timeline’s virtualized row behavior by persisting tool expand/collapse state across remounts and adding a synchronous “measure mounted items” hook to Virtua so recent context group expansions can reflow before the next paint.

Changes:

• Adds a small virtua@0.49.1 patch exposing VirtualizerHandle.measure() and uses it to synchronously remeasure mounted items.
• Makes tool open/close state controllable (open/onOpenChange) in BasicTool and ToolErrorCard, and wires controlled state through MessagePart / MessageTimeline.
• Adds Playwright regression tests covering (1) tool collapse-state persistence under streaming updates and (2) context-group expansion reflow/overlap.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
patches/virtua@0.49.1.patch	Adds measure() to the Solid Virtualizer handle by synchronously remeasuring mounted items.
packages/ui/src/components/tool-error-card.tsx	Adds controlled open state (open, onOpenChange) for error tool cards.
packages/ui/src/components/message-part.tsx	Plumbs controlled tool open state; triggers timeline remeasure on context group expand/collapse.
packages/ui/src/components/basic-tool.tsx	Adds controlled open state support and routes open changes via a unified setter.
packages/app/src/pages/session/message-timeline.tsx	Persists tool open state in the timeline; calls virtualizer.measure() on context group size changes; refactors row rendering around accessors.
packages/app/e2e/regression/session-timeline-context-resize.spec.ts	New regression test asserting no visible overlap/flicker on context group expansion.
packages/app/e2e/regression/session-timeline-collapse-state.spec.ts	New regression test ensuring manual tool collapse persists when later assistant parts stream in.
package.json	Registers the Virtua patch in patchedDependencies.
bun.lock	Adds the Virtua patch entry.

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[lele872]

Regressione from PR #28334 — renderer crash with TypeError: Cannot read properties of undefined (reading 'group')

I'm hitting a renderer crash after updating that maps directly to the refactor in PR #28334 (merged yesterday, May 19).

Stack trace:

TypeError: Cannot read properties of undefined (reading 'group')
at Object.fn (oc://renderer/assets/session-Cj87sRUZ.js:22281:26)
at runComputation (oc://renderer/assets/styles-BzRX2rgK.js:486:22)
at oc://renderer/assets/styles-BzRX2rgK.js:475:9
at runUpdates (oc://renderer/assets/styles-BzRX2rgK.js:582:17)
at oc://renderer/assets/styles-BzRX2rgK.js:472:7
Analysis:

The crash occurs in renderAssistantPartGroup inside message-timeline.tsx (~line 984), where row.group.type is accessed but row is undefined.

The root cause is that PR #28334 removed the Accessor wrapping, switching to direct values. The internal createMemo calls capture row in a stale closure when session state changes (new messages, streaming, session switch), the memo re-executes but row references a stale value that becomes undefined during SolidJS reactive transitions.

Before the refactor, row() was a reactive function that always returned the current value, avoiding this issue.

Question: Does this PR (#28422) cover this specific crash? From the modified files (message-timeline.tsx and message-timeline.data.ts) it looks like it should, but I wanted to confirm since the crash is a hard blocker — the app becomes completely unusable and requires a forced restart.

Thanks for the work!