fix(studio): an audio track can be muted again after it is unmuted - #4977

Reviewed at e02bd463. This is a comment, not an approval. No Codex pass (small PR); this is my own pass.

The main fix holds. An audio-only track now always shows a speaker on its timeline row, and it toggles both ways. A selected <hf-audio-group> finally gets a working mute and unmute in the flat panel, through the group writer, with one undo entry per click. I found no blockers. I have one should-fix, two smaller notes, and one test gap.

1. Should-fix: a track that mixes audio and visual clips now gets a "Hide track" eye, and clicking it silently mutes the audio on that track.
TimelineTrackHeader.tsx:207 / :389 and TimelineTrackPlainHeader.tsx:105-111 now always render the button, with asMute = isAudioOnlyTrack(trackElements) (every clip is audio). On main the gate was visible={!isAudioTrack || isTrackHidden}, and isAudioTrack is some clip is audio (TimelineLanes.tsx:225, unchanged). So on main a mixed row had no control while audible. The PR body says these rows "keep the eye", but they never had one. The click goes to toggleTimelineTrackHidden, which writes data-hidden on every clip on the track (timelineTrackVisibility.ts:209), audio included. The row still shows the Music icon (isAudioTrack), but the button says "Hide" and the audio goes silent in preview and in the render. That is the exact case canHideSelections (timelineInspector.ts:~115-127) refuses for "Hide all": "a control whose label promises visibility". Verified by tracing at the merge-base and at head. The new test "offers the eye on a track mixing audio and a visual clip" pins this behaviour as intended. Options: keep main's gate for mixed rows, name the button for both effects, or hide only the visual clips. Your call, but the current label understates what the click does.

2. Low (the read predates this PR, but it's inside the both-ways claim): after you unmute from the timeline row, or after an undo, the panel still says "Unmute", and the first panel click does nothing.
PropertyPanelFlat.tsx:209 sets hiddenNow = isSelectionHidden(storeHidden, snapshot), and that is an OR with the selection snapshot's data-hidden (propertyPanelFlatClosedGroup.tsx:41-46). The new post-write refresh (usePanelHiddenToggle.ts:36-42) only covers writes made from the panel. Two other paths leave the snapshot stale:

• The row toggle (setElementsHidden, timelineTrackVisibility.ts:~145-176) updates the store and the live DOM, but nothing refreshes the selection.
• The soft undo path (usePreviewPersistence.ts:202-240) syncs only automation. The selection is refreshed only on an iframe reload (useStudioSelectionPublisher.ts:81-102).

I checked this with a scratch test (reverted): store vo-1 hidden:false plus a snapshot with data-hidden="". The panel rendered "Unmute element", and the click called onToggleElementHidden("vo-1", false), which writes nothing new. A group is worse off, because its state comes only from the snapshot, so it stays stale after an undo until the next reload. A fix: for a clip that has a timeline row, read the store alone; or refresh the selection after the row toggle and after a soft undo. I didn't reproduce this in a running studio.

3. Low, design: an audio clip's flat panel now has two mutes next to each other.
The header shows "Mute element" (data-hidden, PropertyPanelFlat.tsx:541-542), and the media section shows the "Muted" toggle (the HTML muted attribute, propertyPanelFlatMediaSection.tsx:367-371). My probe rendered both labels on one clip. The comment this PR deletes cited groups doc §2.1 against exactly that: "two controls that silence a track… differing only in a distinction the author cannot see." I'm not asking you to revert. But the header icon can read "audible" while muted is set, and the reverse, so this needs a decision rather than an accident.

4. Note: a group with mixed member state.
The panel toggle writes only the bus's own data-hidden (usePanelHiddenToggle.ts:23-31). In the runtime, a member is silent if it or its bus is hidden (core/src/runtime/init.ts:456). So one click always lands the bus in a consistent state. Members that were muted on their own stay muted after you unmute the bus, and the panel icon reflects only the bus, with no mixed indicator. That's reasonable bus semantics. I'd just mention it in the doc or in a tooltip.

Test gap: the panel's single-clip unmute isn't exercised. The clip test clicks "Mute element" once on an audible clip. When I made the clip path one-way (!hidden → true), it survived 209/209. One extra test would close this: a hidden clip, "Unmute element", and the call is ("vo-1", false).

Mutation table (8 test files: the 6 changed + timelineTrackVisibility.test.ts + timelineInspector.test.ts; each mutation was reverted after its run)

Things I checked that hold up

• Toggles both ways on the timeline: VisibilityButton calls onToggle(track, !hidden), and hidden is read from the store. The Timeline test goes unmute → mute → unmute → mute.
• One write and one undo entry per click: a clip goes through setElementsHidden → saveProjectFilesWithHistory, and a group through setAudioGroupAttribute with recordEdit. The labels share one owner, hiddenToggleVerb.
• The undo labels match the button. hiddenBefore = !hidden is right for both the track entry and the element entry, and the track entry is unchanged in meaning from main.
• Video: a <video> clip (with or without audio) isn't audio under isAudioTimelineElement, so it keeps the eye, and hide stays distinct from mute. Pure visual rows behave exactly as on main.
• The classic inspector still withholds the toggle on audio, as the PR body says (PropertyPanel.tsx:313). "Hide all" still refuses audio (canHideSelections).
• No handler-level audio refusal sits under the panel's clip path: handleToggleElementHidden only guards by element.
• The post-write refresh re-reads whatever is selected now, not the muted item. M6 and the switch-mid-save test pin that.
• CI at e02bd463: 169 check-runs, 157 success, 12 skipped, 0 failed. The skips are 6 names × 2 (skills, catalog index and GCP BeginFrame, all path-filtered). The Windows studio-1/studio-2 jobs did run on this PR and passed.
• The PR body has the Before/After captures: 3 before and 4 after, on the fixture project. I couldn't open the attachment images, so I haven't checked what they show.

What I ran

• Fetched the head and main into a parked clone. Diffed against merge-base 2cfbe3d2.
• bun install --frozen-lockfile, then built parsers, lint, studio-server and core.
• NODE_ENV=test vitest run on the 8 files above: 209/209 at head.
• One scratch probe test for finding 2, reverted.
• 9 mutations, each one followed by the same 8-file run.
• gh pr checks and the check-runs API for the exact SHA.

— Somu

Approving at e02bd463. This review adds to Somu's comment. The mute now works both ways for an audio track, a clip and a group, and the state is saved to the file.

Checked

• Single clip: the timeline row and the panel both write !hidden. The undo labels come from the same hiddenToggleVerb, so they match the button.
• Group: the panel writes data-hidden on the <hf-audio-group> itself (hidden ? null : ""), and the label follows the state.
• Group with mixed members: only the bus attribute changes. Members muted on their own stay muted, because the runtime silences a member if it or its bus is hidden. I agree with Somu that this is the right bus behaviour; a tooltip could say so.
• Save and reload: both paths write data-hidden into the source file. On reload, timelineSyncHydration.ts:142 reads it back into the track's hidden state, and the group's state comes from the attribute (isSelectionHidden).
• Rows with no audio: visual rows get the same eye as on main (asMute false, and they were visible before). Only the label and icon code moved, into hiddenToggle.tsx.

On Somu's should-fix (a row mixing audio and visual clips): I don't think a running studio can show that row. Every element set into the store goes through normalizeToZones (useTimelinePlayer.ts:90). It packs visual and audio clips into separate display lanes using classifyZone, which is isAudioTimelineElement, the same predicate as isAudioOnlyTrack. Drags are clamped to their own zone (clampTrackToZone). timelineZones.test.ts already pins this case ("drops audio below the visual lanes even when sharing a track index"). So every real row is all audio or all visual, and the old some check and the new every check agree on all of them. The new test "offers the eye on a track mixing audio and a visual clip" builds a row the store never holds. That isn't a blocker. The PR body's "keep the eye" is also inaccurate for those rows, but no user reaches them.

Notes, not blocking

• I agree with Somu's test gap. Nothing exercises unmuting a single clip from the panel (hidden clip, "Unmute element", call is ("vo-1", false)).
• The stale "Unmute" after a row toggle or undo also happens on main. Somu traced it. On main the panel showed the toggle only while it was muted, so it had the same stale read.
• Two mutes now sit together on an audio clip's panel: the header's data-hidden and the media section's HTML muted. That's a product call for the author, as Somu said.

Ran: CI at this head is 88 passed, 6 path-skipped and 0 failed. Locally, the 6 changed test files plus timelineTrackVisibility, timelineInspector and timelineZones pass 232/232.

— Rames