# Performance, Captions, Clip UX, and Resizable Panels

**Date:** 2026-09-21 14:39  
**Task:** Fix timeline performance, caption positioning, clip creation workflow, resizable panels, and deselect UX

## Changes Made

### 1. Performance Optimization (3 files)

**`src/lib/components/VideoPlayer.svelte`**:
- Throttled `handleTimeUpdate` to ~15fps (66ms interval) using `performance.now()` check. Previously every `ontimeupdate` event triggered a reactive cascade through `session.currentTime`.

**`src/lib/components/Timeline.svelte`**:
- **In-flight guard**: Added `detailFetchInFlight` flag to prevent overlapping waveform range extraction subprocess calls. Only one ffmpeg call runs at a time.
- **Increased debounce**: Waveform detail fetch debounce increased from 300ms to 500ms.
- **Playhead-only redraws**: Split the monolithic redraw `$effect` into two:
  - Structural changes (clips, zoom, waveform data, thumbnails) trigger a full `drawMainCanvas()` which saves an `ImageData` snapshot.
  - `session.currentTime` changes trigger a lightweight `drawPlayheadOnly()` that restores the snapshot and draws only the playhead — avoiding the expensive thumbnail/waveform/clip rendering on every time update.
- **Thumbnail load callback**: Registered a callback via `setThumbnailLoadCallback` so thumbnail image loads trigger a targeted redraw instead of relying on the next reactive cycle.

**`src/lib/timeline/thumbnailRenderer.ts`**:
- Added `pendingLoads` Set to prevent creating duplicate `Image` objects for the same path across rapid redraws.
- Added `setThumbnailLoadCallback` API so the Timeline can request a redraw when thumbnails finish loading asynchronously.
- Image `onerror` handler cleans up the pending state.

### 2. Caption Positioning (1 file)

**`src/lib/components/VideoPlayer.svelte`**:
- Added `:global(video::cue)` CSS to center captions at the bottom of the video with a semi-transparent black background, white text, and 16px font size. Used `:global()` to bypass Svelte scoping since `::cue` is a browser-level pseudo-element.
- Added `position: relative` to the `<video>` element to scope cue rendering.

### 3. Pending In-Point Preservation (1 file)

**`src/lib/stores/clips.svelte.ts`**:
- Modified `selectClip()` to only clear `pendingInPoint` when `id !== null` (selecting a specific clip). When `id === null` (deselecting via empty timeline click), the pending in-point is preserved. This fixes the workflow: press I → click timeline to seek → press O to complete the clip.

### 4. Resizable Panels (3 files)

**`src/App.svelte`**:
- Wrapped `<Timeline />` and `<ClipList />` in a new `.timeline-clip-area` container with flex column layout.
- Added a 5px `.resize-handle` divider between them with `cursor: ns-resize` and accent color on hover/active.
- Implemented `handleResizeStart/Move/End` with `mousedown`/`mousemove`/`mouseup` on `<svelte:window>` to drag-resize. Timeline height is clamped between 80px and (total - 60px).
- `timelineHeight` stored in local `$state` (defaults to 180px).

**`src/lib/components/Timeline.svelte`**:
- Changed `.timeline-container` from `height: 160px` to `flex: 1; min-height: 80px` so it fills the space given by the parent.
- `.timeline-wrapper` set to `height: 100%`.

**`src/lib/components/ClipList.svelte`**:
- Changed from `max-height: 200px` to `height: 100%; min-height: 40px` with `box-sizing: border-box`.

### 5. Deselect UX (1 file)

**`src/lib/components/ClipList.svelte`**:
- Added `handleContainerClick` on the `.clip-list` div that calls `selectClip(null)` when clicking empty space (not a child clip row).
- Replaced the single "Export All" button with an `.action-buttons` row containing a conditional "Deselect" button (shown only when a clip is selected) alongside "Export All".
- Changed `role="button"` to `role="option"` on clip rows and `role="listbox"` on the container for better semantics.

## Build Status

- Frontend: builds cleanly (0 errors)
- Rust: compiles with only pre-existing unused variant warnings

## Lessons Learned

- Svelte's scoped CSS doesn't apply to `::cue` pseudo-elements since they're browser-level shadow constructs — `:global()` wrapper is needed.
- Canvas `getImageData`/`putImageData` is an effective technique for "snapshot and overlay" rendering patterns, avoiding expensive full redraws when only one layer (like a playhead) changes.
- `ontimeupdate` fires at varying rates across browsers/engines; throttling to a target frame rate prevents reactive cascades from dominating the UI thread.
- The `pendingInPoint` clearing bug was a subtle consequence of `selectClip(null)` being used for two different intents (deselecting a clip vs. resetting all clip state).