[Bug]: Paused sidebar video reloads twice and loses playback position after read-only commands#23089

A local MP4 left paused in the sidebar repeatedly returns to loading as unrelated read-only commands finish. If the clip was paused after playback, it also loses the user's position: a video paused at 5 seconds resets to the 0.1-second first-frame preview even though its bytes have not changed.

Before submitting

• I searched existing issues and found no matching paused-video report; related HTML preview reports are linked below.
• I included reproduction steps, environment and before/after evidence.

Area

apps/web (shared by desktop)

Steps to reproduce

1. Have an agent link a local MP4 in a thread, then open it in the sidebar preview.
2. Leave it paused at 0:00. For the playback-position case, play it, seek to about 5 seconds and pause.
3. In the same thread, have the agent run read-only commands against another file, such as Get-Content .\README.md -TotalCount 5 on Windows.
4. Watch the preview as commands finish. Repeat with another read-only command. Keep the MP4 unchanged.

Expected behavior

The preview checks for workspace changes while preserving an unchanged video's loaded state and paused playback position. Actual changes to the video still update the preview.

Actual behavior

At zero, the video repeatedly shows loading while left paused. At a nonzero paused position, it reloads and resets to 0.1 seconds.

Instrumenting the actual FilePreviewPanel with the real asset RPC and streamed MP4 showed two source changes and two emptied/loadstart cycles per controlled completed-command update. Re-rendering with the same mutation id caused no reload. The original MP4's SHA256 stayed unchanged.

Impact

Minor bug or occasional failure: repeated loading and loss of the user's place while reviewing a generated video alongside agent work.

Version or commit

• Originally observed: T3 Code Nightly desktop 0.1.1-nightly.20261020.1792535967237.
• Instrumented baseline reproduced at d27c6067a9162150f654e38f455a7a936ef79178.

Environment

Windows 11 Pro, build 26200; a locally generated MP4 that decodes without errors.

Before/after evidence

Both sides start paused at 5 seconds. The same command-completion update is applied at 1.5 seconds into the recording. Before: two reloads and a reset to 0.1 seconds. After: zero reloads and the position stays at 5 seconds.

https://github.com/user-attachments/assets/f38df741-de92-4228-b47c-8c7581406921

The recording uses controlled command-completion fixtures in the real preview component, with real media RPC and file streaming. It is not a full provider turn. The installed Nightly was not rebuilt, and mobile was not exercised.

Investigation and proposed repair

Completed commands change workspaceMutationId, including read-only commands. The video source changes immediately with the workspace-revision suffix, then changes again when the refreshed signed URL arrives. For paused video, both changes reload the element.

Related: #20913 and Ideas #20927. The maintainer-account response on #20913 describes HTML refresh as intentional and routes a change to Ideas. This video failure shares that refresh mechanism; this report asks for triage of its effect on paused playback and loss of position. No media-specific approval is assumed.

A proposed repair is available on NikitaMGrimm:fix/paused-media-refresh, commit 7221dba531f0690b43bdb0560c8f02a15d1c71c4. It adds optional per-file revision metadata and preserves loaded sidebar video/audio when the file is unchanged. It keeps actual file updates, URL renewal, retry and older-server compatibility; HTML/PDF behavior is unchanged.

The fixed reproduction also stays loaded at zero. Actual in-place edits and atomic replacements reload paused video once, and a change during playback waits until pause. The focused regressions cover file revisions, source pinning, failure recovery and compatibility; check:changed passed on that head, including 46 tests, using a Windows command-launch workaround. No PR is open.

Workaround

To keep a clip paused at a chosen frame while agent commands continue, open it in an external player and pause there. The demonstrated reset affects paused video; we have not reproduced it during active playback.