chat-summaries/2026-09-21_14-39-perf-captions-resize-ux-summary.md

# 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).
chore: stage all pending work — caption styling, media server, processing modal, docs, summaries Includes: - Extended caption styling (font, shadow, dimmed color, bg toggle) - Media server, subtitle downloader, VTT parser, processing modal - Waveform tiers, thumbnail/timeline improvements, transport controls - Hybrid download model, dependency management, clip export enhancements - 21 chat summaries, 2 implementation plans, 2 design specs Co-authored-by: Cursor <cursoragent@cursor.com> 2026-09-22 10:48:16 -04:00			`# 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).