Diff panel scroll jitters and snaps to the next file on a very large diff until the thread is switched #29967

What happened

Opened the Changes diff panel in a thread on a diff of about +31k lines. Scrolling down with a trackpad intermittently jitters and won't scroll freely. After the jitter the view snaps to the next file in the diff. It starts after the panel has been open for a while and isn't consistent. Turning line wrapping off did not stop it. Switching to another thread made it disappear.

Diagnosis

Unconfirmed. The logs show no renderer crashes and no server stalls, so it appears to be client-side. The diff panel uses the virtualized CodeView from @pierre/diffs 1.6.1. StyledDiffCodeView.tsx:242-253 sets fixed item metrics (header 32px, hunk separator 24px, padding 8px). StyledDiffCodeView.tsx:15-19 already notes that the virtualizer "repeatedly correct[s] the scroll position" in split + wrap mode. Wrap isn't the trigger here. Remaining candidates:

• Height corrections when estimated and measured item heights differ, made worse by collapsed files.
• Lazily loaded file patches (useReviewFilePatches, lazySource) resizing items mid-scroll.
• State building up in the long-lived viewer. The codeViewKey remount (DiffPanel.tsx:~1384) clears it, which would explain why switching threads fixes it.

Steps to reproduce

1. Open a thread with a very large diff (about +31k lines, many files).
2. Open the Changes panel in split layout with some files collapsed.
3. Scroll down with a trackpad and keep the panel open for a while.
4. Intermittently the scroll jitters and snaps to the next file.
5. Toggling line wrapping off doesn't help. Switching to another thread clears it.

Not yet tested: unified layout, all files expanded, or a smaller diff.

Version

0.1.6 (desktop app, the latest stable release, on the same machine as the report).

Environment

macOS (Darwin 27.0.0) arm64, Node v24.22.1, T3 Code Desktop 0.1.6 against the local server 0.1.6. Renderer working set about 520–560 MB, peak 657 MB.

Evidence

No relevant errors in `desktop.trace.ndjson`, `renderer-history.ndjson`, or `event-loop-stalls.ndjson`. The only stall is from Oct 20 and is unrelated. No `server-child` crash logs from this session.

Related issues

No duplicate found. Nearby but different: #28160 (horizontal trackpad scroll triggers back/forward), #11571 (diff tab loses scroll position when switching threads), #17674 (collapse all not applied to unloaded files in large diffs).

Fix applied or workaround

Nothing was run on the machine. Workaround: switch to another thread and back, which remounts the viewer.

Filed by

Claude Code (model claude-sonnet-5-5) via the T3 Code triage playbook.