@remotion/studio: Fix multiple marquee selection

[JonnyBurger]

Summary

• allow Cmd/Ctrl+marquee to extend the current keyframe and easing selection
• preserve the existing selection from pointer-down through the marquee drag
• avoid duplicate selections and add regression coverage

Testing

• bun test packages/studio/src/test/clear-selection-on-pointer-down.test.ts packages/studio/src/test/timeline-selection.test.ts
• bun run build
• bun run stylecheck

Closes #9379

[vercel]

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Project	Deployment	Actions	Updated (UTC)
bugs	Ready	Preview, Comment	Jul 23, 2026 9:07am
remotion	Ready	Preview, Comment	Jul 23, 2026 9:07am

[pullfrog]

✅ No new issues found.

Reviewed changes — Allows Cmd/Ctrl+marquee drag to extend the existing timeline selection (keyframes, easings, sequences) instead of replacing it, while preserving Shift+marquee's existing no-op behavior.

• Add extendTimelineMarqueeSelection — merges a current selection with a marquee-drag selection, filtering incompatible items and deduplicating by selection key.
• Wire Cmd/Ctrl+marquee through useTimelineMarqueeSelection — captures selectedItems at pointer-down for use as the extending base, and applies extendTimelineMarqueeSelection when the marquee was started with a modifier key held.
• Preserve selection on Cmd/Ctrl pointer-down — shouldClearSelectionOnPointerDown now accepts ctrlKey/metaKey and returns false when either is true, preventing selection loss before an extending marquee starts.
• Add regression tests — unit coverage for extend marquee merging, empty-marquee preservation, duplicate removal, and modifier-key blocking of selection clear.

The implementation is clean: the selectionBeforeMarquee snapshot captures the selection at pointer-down time, the extendSelection flag gates the extend path, and the merge function correctly handles empty marquees, incompatible kinds, and duplicates. The selectedItems dependency addition in useCallback is a standard pattern with negligible cost.

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[pullfrog]

✅ No new issues found.

Reviewed changes since prior review — commit 4c3d07d tightens the shouldClearSelectionOnPointerDown parameter types from optional (ctrlKey?: boolean) to required-nullable (ctrlKey: boolean | null) and switches from truthy modifier-key checks to strict === true comparisons. Tests are updated to pass explicit null values.

• Stricter parameter typing in shouldClearSelectionOnPointerDown — ctrlKey/metaKey are now required boolean | null fields, forcing callers to be explicit. The === true check is equivalent to the prior truthy check for boolean inputs and prevents inadvertent truthy matches on non-boolean values.
• Test cleanup — all test callsites now pass ctrlKey: null, metaKey: null explicitly, making modifier-key intent visible at each assertion.

The callers (Preview.tsx, EditorContent.tsx) already pass standard DOM PointerEvent objects whose .ctrlKey/.metaKey are always boolean, so the type change requires no caller refactoring and the === true check produces identical runtime behavior.

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