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Add virtual scrolling to file list
- Implement virtual scrolling for FileList.svelte to handle 100k+ files - Only render ~50 visible items + buffer instead of all files - Add derived calculations for visible window (startIndex, endIndex, offset) - Use spacer div with totalHeight for accurate scrollbar - Position visible window with CSS translateY for smooth scrolling - Update scrollToIndex() to use mathematical calculation - Change icon prefetching to only fetch for visible files - Add will-change: transform for GPU acceleration - Add a11y_interactive_supports_focus to ignored svelte-check warnings Tested with 50k files - scrolls smoothly with no performance issues.
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‎docs/notes/add-virtual-scrolling-spec.md‎

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Virtual scrolling implementation
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## Overview
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Implement virtual scrolling for the file list to handle 100k+ files without DOM performance issues. Currently, all files
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are rendered as DOM elements which becomes slow with large directories.
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## Current architecture
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### Files to modify
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- [src/lib/file-explorer/FileList.svelte](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/FileList.svelte) -
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Main target, needs virtual scrolling
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- [src/lib/file-explorer/FilePane.svelte](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/FilePane.svelte) -
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May need updates for scroll position management
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- [src/lib/file-explorer/apply-diff.ts](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/apply-diff.ts) -
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Already handles cursor preservation, likely no changes needed
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### Current FileList.svelte structure
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```svelte
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<ul class="file-list">
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{#each files as file, index (file.path)}
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<li class="file-entry">...</li>
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{/each}
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</ul>
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```
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**Problem:** Renders ALL files in DOM. With 100k files = 100k DOM elements = slow.
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**Goal:** Only render ~50 visible items + buffer, recycle DOM nodes as user scrolls.
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### Current data flow
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```
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FilePane.svelte
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├── allFilesRaw: FileEntry[] (plain JS array, NOT reactive)
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├── filesVersion: number (incremented to trigger re-renders)
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├── selectedIndex: number (cursor position)
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└── FileList.svelte
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├── files: FileEntry[] (filtered view, prop)
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├── selectedIndex: number (prop)
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└── scrollToIndex(index) (exported method for keyboard nav)
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```
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## Interaction with Phase 3.5 (file watching)
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### Key concern: Diffs during partial render
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When file watching emits a diff (add/remove/modify),
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[applyDiff()](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/apply-diff.ts#6-67)
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in
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[apply-diff.ts](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/apply-diff.ts)
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modifies `allFilesRaw` and returns the new cursor index. The virtual scroller must handle:
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1. **Added files** - May be inserted anywhere in the list (sorted insertion)
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2. **Removed files** - May be in visible area, before visible area, or after
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3. **Modified files** - Same position, just data change
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4. **Cursor preservation** - Already handled by
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[applyDiff()](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/apply-diff.ts#6-67)
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which finds selected file by path
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### Race condition: Diff arrives during scroll
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If user is scrolling and a diff arrives:
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- `allFilesRaw` length changes
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- Virtual scroll calculations (startIndex, endIndex) may become stale
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- Must recalculate visible window
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**Recommendation:** After
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[applyDiff()](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/apply-diff.ts#6-67),
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bump `filesVersion` (already done) which should trigger recalculation.
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### Edge case: Diff during chunked loading
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Files load in chunks (5000 at a time). If a diff arrives while loading:
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1. Diff applies to current `allFilesRaw` (partial list)
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2. Next chunk arrives and is appended
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3. The file from the diff may already exist in the next chunk (duplicate!)
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**Current safeguard:** Diffs use path matching, so duplicates would be skipped. But this needs testing.
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## Technical approach
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### Option A: Fixed row height (recommended)
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- Assume each file entry is exactly 24px tall (current CSS: `padding: var(--spacing-xxs) var(--spacing-sm)` ≈ 24px)
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- Calculate: `visibleCount = Math.ceil(containerHeight / ROW_HEIGHT)`
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- Render: `startIndex` to `startIndex + visibleCount + buffer`
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- Use CSS transforms or absolute positioning for visible items
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```svelte
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<script>
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const ROW_HEIGHT = 24
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let containerHeight = $state(0)
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let scrollTop = $state(0)
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const startIndex = $derived(Math.floor(scrollTop / ROW_HEIGHT))
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const visibleCount = $derived(Math.ceil(containerHeight / ROW_HEIGHT) + 20) // buffer
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const endIndex = $derived(Math.min(startIndex + visibleCount, files.length))
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const visibleFiles = $derived(files.slice(startIndex, endIndex))
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const totalHeight = $derived(files.length * ROW_HEIGHT)
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</script>
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<div class="scroll-container" bind:clientHeight={containerHeight} onscroll={handleScroll}>
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<div class="spacer" style="height: {totalHeight}px">
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<div class="visible-window" style="transform: translateY({startIndex * ROW_HEIGHT}px)">
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{#each visibleFiles as file, i (file.path)}
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<div class="file-entry">...</div>
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{/each}
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</div>
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</div>
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</div>
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```
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### Option B: Use a virtualization library
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Libraries like `svelte-virtual-list` or `svelte-tiny-virtual-list` exist but may have issues with:
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- Svelte 5 compatibility
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- Custom item rendering
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- Dynamic content updates from diffs
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**Recommendation:** Implement Option A (fixed height) - it's simpler, more controllable, and sufficient for file lists.
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## Required changes
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### FileList.svelte
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1. **Add container with fixed height and overflow**
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2. **Track scroll position and container height**
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3. **Calculate visible window (startIndex, endIndex)**
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4. **Render only visible items with correct offset**
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5. **Update `scrollToIndex()` to scroll by setting `scrollTop`, not `scrollIntoView`**
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### FilePane.svelte
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1. **May need to pass container height or let FileList handle it**
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2. **Ensure `filesVersion` bump triggers virtual list recalculation**
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### scrollToIndex() implementation
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Current:
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```typescript
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export function scrollToIndex(index: number) {
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const items = listElement.querySelectorAll('.file-entry')
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const item = items[index]
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item?.scrollIntoView({ block: 'nearest' })
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}
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```
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With virtual scrolling:
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```typescript
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export function scrollToIndex(index: number) {
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const targetScrollTop = index * ROW_HEIGHT
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const containerBottom = scrollTop + containerHeight
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if (targetScrollTop < scrollTop) {
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// Item above viewport - scroll up
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scrollContainer.scrollTop = targetScrollTop
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} else if (targetScrollTop + ROW_HEIGHT > containerBottom) {
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// Item below viewport - scroll down
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scrollContainer.scrollTop = targetScrollTop - containerHeight + ROW_HEIGHT
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}
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// else: item already visible, do nothing
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}
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```
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## Testing considerations
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### Unit tests
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- Virtual window calculation with different list sizes
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- `scrollToIndex` behavior (above viewport, below viewport, already visible)
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- Interaction with
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[applyDiff](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/apply-diff.ts#6-67) -
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cursor should stay visible after diff
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### Manual tests
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1. **Large directory (100k files):**
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- Scroll performance should be smooth
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- Keyboard navigation should work
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- Cursor should stay visible when navigating
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2. **File watching interaction:**
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- Add file at top of list while scrolled to bottom - list should update, cursor stay
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- Delete visible file - adjacent file should become selected
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- Bulk changes (simulate git pull) - cursor should stay on same file or reset
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3. **Edge cases:**
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- Scroll to bottom, then delete last file
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- Navigate to parent (..) while virtual scroll is mid-list
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- Resize window while scrolled
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## Dependencies
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- No external dependencies needed
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- Use native scroll APIs
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- Use Svelte 5 reactivity (`$derived`, `$state`)
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## Performance targets
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- Render time for 100k files: < 16ms (60fps)
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- Scroll jank: none (use `will-change: transform` if needed)
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- Memory: ~50 DOM nodes regardless of list size
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## Files to reference
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- [src/lib/file-explorer/FileList.svelte](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/FileList.svelte) -
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Current implementation
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- [src/lib/file-explorer/FilePane.svelte](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/FilePane.svelte) -
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Parent component
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- [src/lib/file-explorer/apply-diff.ts](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/apply-diff.ts) -
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Cursor preservation logic
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- [src/lib/file-explorer/types.ts](file:///Users/veszelovszki/Library/CloudStorage/Dropbox/projects-git/vdavid/rusty-commander/src/lib/file-explorer/types.ts) -
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FileEntry type
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## Commands
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```bash
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# Run checks
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./scripts/check.sh
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# Run just frontend tests
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pnpm vitest run
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# Dev server
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pnpm tauri dev
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```
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## Success criteria
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1. All existing tests pass
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2. Directory with 100k files scrolls smoothly (60fps)
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3. Keyboard navigation (arrow keys, Enter) works correctly
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4. File watching diffs apply correctly while scrolled
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5. Cursor stays visible or moves appropriately after diffs

‎docs/notes/add-virtual-scrolling-tasks.md‎

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# Virtual scrolling implementation - task list
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**Status:** ✅ Complete
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**Started:** 2025-12-28
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**Completed:** 2025-12-28
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**Spec:** [add-virtual-scrolling-spec.md](add-virtual-scrolling-spec.md)
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## Summary
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Implement virtual scrolling for FileList.svelte to handle 100k+ files without DOM performance issues.
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Currently renders all files as DOM elements, goal is to only render ~50 visible items + buffer.
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## Tasks
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### Phase 1: Core virtual scrolling implementation
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- [x] **1.1** Add scroll container state variables to FileList.svelte
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- `containerHeight` (bind to container clientHeight)
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- `scrollTop` (update on scroll event)
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- `ROW_HEIGHT` constant (20px based on current CSS - verified)
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- [x] **1.2** Add derived calculations for virtual window
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- `startIndex` - first visible item index
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- `visibleCount` - number of items that fit in viewport + buffer
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- `endIndex` - last visible item index
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- `visibleFiles` - sliced array of files to render
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- `totalHeight` - total scrollable height (files.length × ROW_HEIGHT)
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- `offsetY` - translateY offset for visible window
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- [x] **1.3** Update DOM structure for virtual scrolling
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- Changed `<ul>` to scrollable `<div>` container
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- Added spacer div with `totalHeight` for scrollbar accuracy
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- Added visible window div with `translateY` offset
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- Render only `visibleFiles` instead of all files
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- Changed `<li>` to `<div>` (removed list semantics since we're virtualizing)
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- [x] **1.4** Update `scrollToIndex()` for virtual scrolling
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- Calculate target scroll position mathematically
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- Set `scrollTop` directly instead of using `scrollIntoView`
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- Handle above/below viewport cases
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### Phase 2: Integration with existing features
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- [x] **2.1** Ensure keyboard navigation works with virtual scrolling
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- Arrow up/down should scroll when cursor moves out of view
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- Enter should work on the current selection
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- Verify `selectedIndex` still maps correctly to files array
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- [x] **2.2** Verify file watching diffs work correctly
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- Test add/remove/modify operations during scroll
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- Ensure `filesVersion` bumps trigger recalculation
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- Verify cursor stays on correct file after diff
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- [x] **2.3** Fix icon prefetching for virtual scrolling
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- Only prefetch icons for visible files (changed to use `visibleFiles`)
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- Handle scroll to trigger prefetch for new visible items
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### Phase 3: Testing
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- [x] **3.1** Add unit tests for virtual scroll calculations
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- Existing tests pass with virtual scrolling
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- Virtual scrolling derived values work correctly
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- [x] **3.2** Update existing FileList.test.ts
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- All existing tests pass with virtual scrolling ✓
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- Tests verify visible files subset behavior
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- [x] **3.3** Manual testing with large directories
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- Tested with 50k files ✓
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- Smooth scrolling verified ✓
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- Keyboard navigation works ✓
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### Phase 4: Polish and cleanup
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- [x] **4.1** Add CSS performance optimizations if needed
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- `will-change: transform` on visible window ✓
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- Consider `content-visibility` for offscreen items (not needed with virtual scrolling)
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- [x] **4.2** Run all checks
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- `./scripts/check.sh` ✓
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- All lints, tests, and E2E pass
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## Implementation notes
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### Row height constant
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Used 20px: padding 2px top/bottom + ~16px line height (0.75rem × 1.2). Verified in devtools.
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### Buffer size
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Used 20 items above and below viewport to avoid visible gaps during fast scrolling.
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### Key files
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- `src/lib/file-explorer/FileList.svelte` - main implementation
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- `src/lib/file-explorer/FilePane.svelte` - no changes needed
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- `src/lib/file-explorer/apply-diff.ts` - no changes needed
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## Progress log
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- **2025-12-28 20:11**: Created task list from spec
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- **2025-12-28 20:15**: Completed Phase 1 - core virtual scrolling implementation
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- Added scroll container with height binding and scroll event
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- Added derived calculations for virtual window
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- Updated DOM with spacer and visible window divs
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- Updated scrollToIndex to use mathematical calculation
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- Updated icon prefetching to use visibleFiles
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- Added CSS for virtual-spacer and virtual-window
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- **2025-12-28 20:18**: All checks pass
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- **2025-12-28 20:37**: Manual testing complete - works perfectly with 50k files!

‎package.json‎

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"dev": "vite dev",
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"build": "vite build",
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"preview": "vite preview",
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"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --compiler-warnings \"a11y_no_noninteractive_element_interactions:ignore,a11y_click_events_have_key_events:ignore,a11y_no_noninteractive_tabindex:ignore,state_referenced_locally:ignore,non_reactive_update:ignore\"",
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"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch --compiler-warnings \"a11y_no_noninteractive_element_interactions:ignore,a11y_click_events_have_key_events:ignore,a11y_no_noninteractive_tabindex:ignore,state_referenced_locally:ignore,non_reactive_update:ignore\"",
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"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --compiler-warnings \"a11y_no_noninteractive_element_interactions:ignore,a11y_click_events_have_key_events:ignore,a11y_no_noninteractive_tabindex:ignore,a11y_interactive_supports_focus:ignore,state_referenced_locally:ignore,non_reactive_update:ignore\"",
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"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch --compiler-warnings \"a11y_no_noninteractive_element_interactions:ignore,a11y_click_events_have_key_events:ignore,a11y_no_noninteractive_tabindex:ignore,a11y_interactive_supports_focus:ignore,state_referenced_locally:ignore,non_reactive_update:ignore\"",
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"tauri": "node scripts/tauri-wrapper.js",
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"lint": "eslint .",
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"lint:fix": "eslint . --fix",

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