@remotion/studio: Add Shift/Cmd timeline multi-select

[JonnyBurger]

Summary

• add modifier-aware timeline selection interactions for rows and keyframes
• implement Shift+click contiguous range selection using registered visible timeline items
• implement Cmd/Ctrl+click toggle selection behavior
• keep selection constrained to one item type at a time (row or keyframe)
• wire pointer handlers to pass modifier state into selection logic
• add unit tests covering range/toggle and mixed-type fallback behavior

Notes

• if the current anchor type does not match the clicked item type, Shift+click falls back to selecting only the clicked item

Closes #7800

[vercel]

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Project	Deployment	Actions	Updated (UTC)
bugs	Ready	Preview, Comment	May 31, 2026 1:43pm
remotion	Ready	Preview, Comment	May 31, 2026 1:43pm

[pullfrog]

Reviewed changes — modifier-aware multi-select interactions (Shift+click range, Cmd/Ctrl+click toggle) for timeline rows and keyframes, plus registration of visible items to establish selection order.

• Add TimelineSelectionInteraction and TimelineSelectionState types — model modifier keys and anchor/selection state.
• Implement getTimelineSelectionAfterInteraction — pure selection-state reducer handling range select, toggle, and plain-click fallback with type-constrained semantics.
• Add registerSelectableItem to TimelineSelectionProvider — tracks visible items in mount order via refs for range-selection ordering.
• Wire modifier keys through pointer handlers — TimelineKeyframeDiamond, TimelineRowChrome, and TimelineSequence now pass shiftKey/metaKey/ctrlKey into selection logic.
• Add unit tests — cover toggle and range selection for rows, plus mixed-type anchor fallback.

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Note: 1 inline comment(s) dropped because they did not anchor to lines inside the PR diff:

• packages/studio/src/components/Timeline/TimelineRowChrome.tsx:94 (RIGHT) — line 94 (RIGHT) is not inside a diff hunk

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using Kimi K2 ｜ 𝕏}