@remotion/studio: Refactor timeline selection state for multiple items

[JonnyBurger]

Summary

Refactors the timeline selection state from a single selectedItem to a selectedItems array so 0..n items can be selected at once. Existing single-click behaviour is preserved; shift / cmd / ctrl + click now toggles items into or out of the selection.

Details

• Selected items must all share the same type (rows or keyframes). Adding a differently-typed item replaces the selection, and a runtime assertion enforces the invariant.
• selectItem now accepts an optional {additive} flag wired into the click handlers in TimelineRowChrome, TimelineSequence, and TimelineKeyframeDiamond.
• isSelected and containsSelection consider every selected item.
• Backspace fires one delete API call per selected item in parallel (Promise.all) and clears the selection.

Split out from #7782. Closes #7799.

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Project	Deployment	Actions	Updated (UTC)
bugs	Ready	Preview, Comment	May 29, 2026 3:32pm
remotion	Ready	Preview, Comment	May 29, 2026 3:32pm

[pullfrog]

Reviewed changes — Refactored timeline selection state from a single selectedItem to a selectedItems array with additive selection support (shift/cmd/ctrl + click), parallel batch deletion, and cleanup of the now-unused keyframed-value interpolation path.

• Multi-select state management in TimelineSelection.tsx — Replaced selectedItem with selectedItems, added SelectItemOptions for additive toggling, and optimized isSelected with a Set of selection keys.
• Batch deletion in TimelineDeleteKeybindings.tsx — Backspace now fires deleteSelectedTimelineItem in parallel via Promise.all and clears the selection afterward.
• Additive click handlers — TimelineRowChrome, TimelineSequence, and TimelineKeyframeDiamond all pass {additive: e.shiftKey || e.metaKey || e.ctrlKey} to onSelect.
• Removed dead keyframed-value code — Deleted interpolate-keyframed-status.ts, its test file, and TimelineKeyframedValue.tsx; removed frame parameter from computeEffectiveSchemaValuesDotNotation and wrapInSchema.
• Cleaned up keyframeDisplayOffset plumbing — Removed the prop from all expanded row/field components since keyframed values are no longer rendered inline.

ℹ️ Nitpicks

• CanUpdateSequencePropStatusKeyframed is still exported from packages/core/src/use-schema.ts and re-exported via Internals, but nothing in the codebase references it anymore after this PR. Consider removing the type and its export to keep the surface clean.
• assertSameType in TimelineSelection.tsx (line 105) is defensive but effectively unreachable through normal UI interaction because selectItem already replaces the selection when a differently-typed item is added. It doesn't hurt, but it's redundant.

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

[pullfrog]

Reviewed changes (incremental) — The follow-up commit 2de6be2 reverts the user-facing additive-selection UI (shift/cmd/ctrl + click) from TimelineRowChrome, TimelineSequence, and TimelineKeyframeDiamond while keeping the internal refactor intact. This is a clean, conservative approach: the state layer now supports multiple selections and batch deletion, but the UI continues to behave as single-select until a future PR enables multi-select interactions.

• Revert is complete — No dangling SelectItemOptions imports or additive handler logic remains in the three UI components.
• Types remain consistent — useTimelineRowSelection and useTimelineKeyframeSelection still expose onSelect(options?), but the consumers' () => void prop types are assignable in TypeScript (optional parameters are compatible).
• No regressions introduced — The revert only removes the modifier-key click handling; all internal state, selectedItems array logic, and Promise.all batch deletion remain unchanged.

  ^{｜ View workflow run ｜ Using Kimi K2 ｜ 𝕏}