fix(studio): shift+click on a group member removes it for good

[miguel-heygen]

What

In the Studio preview, shift+click on a member of a group selection now removes it, and it stays removed. Before, the member came back a moment later, so a group could never be trimmed by clicking. The same happened when a marquee caught just one member of a live group.

Why

The timeline keeps the selection set, and it syncs that set back onto the canvas. When shift+click shrank a group to one element, the canvas announced only a new anchor and left the timeline's set at two members. About 70 ms later the sync saw two wanted and one on the canvas, and re-selected both (apply nextGroup:1, then timeline-sync wanted:2 had:1, then marquee hits:2 in the hf-select debug log). A move right after that dragged the removed element too.

The mirror guessed intent from the group size: a one-element group whose anchor is already in the set was treated as a late async primary and kept the old set. A shift toggle or a marquee that ends at one element looks the same by size.

Related work

Refs #3146, which introduced the set-preserving rule this narrows.

How

The caller now says when its group is the whole new selection. applyDomSelection passes that for a shift toggle, and applyMarqueeSelection always does, so announceTimelineSelection publishes the new set in the same step as the canvas change. There is no race left to lose, and no timer. A plain single selection of an element already inside the live set still keeps the set, as before.

Test plan

• Unit tests added/updated: useDomSelection.test.ts now reads back the set it last published, the way the timeline store does. Two new cases (shift+click removes a member, one-element marquee over a group) fail on main with the set still holding both ids, and pass here. A third case pins the old rule: re-announcing one member keeps the set; it fails if the mirror always replaces the set.
• Manual testing performed on a fixture project with the timeline open: group two elements, shift+click one member (the timeline and inspector show one element at 150 ms and at 1 s, no sync re-select in the log), shift+click a non-member (added), shift+click it again (removed), move what is left (only that element's position is written), undo (file reverts). A one-element marquee over a group of two leaves one.
• Documentation updated (if applicable)
• Comments follow CONTRIBUTING.md "Comments"

Before

Group of two, then shift+click on Tiny. A second later both are selected again.

After

Same gesture. Tiny is removed and stays removed; shift+click on Headline adds it.

[jrusso1020]

Approve at c75cb1f8.

The cause in the body checks out against the code. A shift toggle that shrank a group to one element looked, by size alone, like a late async primary inside the live set, so the mirror kept the old set, and the timeline sync re-selected the removed member. The explicit replaceSet from the caller fixes that without a timer.

Callers checked. I checked every caller of announceTimelineSelection in useDomSelection.ts:

• applyDomSelection (l.183) passes isAdditiveSelection. Both shift-add and shift-remove state the full group, so replacing the set is right. With preserveGroup plus additive, replaceDomEditGroupSelection also returns the full group.
• applyMarqueeSelection (l.514) always passes true. The additive branch seeds the current group, and the non-additive branch is the whole marquee, so both are complete.
• refreshDomEditGroupSelectionsFromPreview (l.430) and the clear paths (l.115, l.187) keep the old behaviour. That is correct for the refresh, which is the late-re-resolve case the preserve rule exists for.

Mutation check. With useDomSelection.ts and domSelectionTimelineMirror.ts reverted to main, the two new cases fail: "shift+click removes a member" and "a marquee catches one member of a live group". The re-announce case that pins the old rule passes on both.

Tests. 1074/1074 pass across src/hooks in studio, and studio tsc --noEmit is clean.

Non-blocking. The new mirror comment lists "timeline sync" among the callers that state the group in full, but no timeline-sync path passes replaceSet. Only the shift toggle and the marquee do. It is worth trimming so the next reader doesn't go looking for it.

— Rames