refactoring-select-from-list

name	refactoring-select-from-list
description	Domain knowledge for the selectFromList webview component in vscode-textmate. Use when modifying, debugging, or extending the fuzzy-search selector UI — covers architecture, data flow, key files, and known constraints.

selectFromList Architecture

A webview-based fuzzy-search selector that replaces VS Code's native QuickPick for richer interaction (multi-select, range select, alternate actions).

Key Files

File	Role
src/selectFromList.js	Extension-side provider — creates/reuses webview, posts items, resolves promises
src/selectFromList/main.js	Webview-side — List class (virtual scrolling, selection), FuzzySearch class (TextMate-style ranking)
src/selectFromList/index.html	HTML shell with inline CSS, search input, CSP nonces
src/selectFromList.json	Manifest fragment (contributes views, commands, configuration)

Data Flow

Command invokes showSelectFromList(items, options)
  → chooseItems() on provider
    → createWebviewPanel() (reuses if alive) or sidebar path
    → reveal() panel (so it's visible before init)
    → await _isReady (resolves when webview posts "ready")
    → postMessage({ type: "init", items, requestId })
  → webview receives init
    → pre-lowercases labels (_lowerLabel, _lowerDescription)
    → computeVisible() → render() (virtual scrolling)
    → focus search input
  → user types → 30ms debounce → computeVisible() → render()
  → user submits (Enter / double-click)
    → postMessage({ type: "submit", indexes, requestId })
  → provider resolves promise with selected items

FuzzySearch (TextMate port)

• rank(filter, candidate, candidateLower, out) — entry point, filter is already lowercase
• calculateRank(lhs, rhs, rhsLower, out) — matrix-based scoring from TextMate's ranker.cc
• rankFile(filter, filename, directory, filenameLower, directoryLower) — tries filename first, falls back to full path
• isSubset(needle, haystackLower) — fast pre-check before full ranking
• Pre-allocated typed arrays (_matrix, _first, _last, _capitals) avoid GC pressure

Performance Characteristics

• Virtual scrolling — only ~60 DOM nodes regardless of list size (20-row overscan)
• Incremental filtering — typing more chars narrows from previous matches, not full list
• Pre-lowercased labels — toLowerCase() called once at init, not per-char in hot loop
• Panel reuse — retainContextWhenHidden: true, panel is reused across opens

Constraints

• The FuzzySearch.calculateRank algorithm is a TextMate port — modify with extreme care
• requestId prevents stale responses from previous invocations
• close() disposes panel (panel path) or hides sidebar — different teardown per render mode
• The _isReady promise pattern: created fresh when panel/sidebar is new, stays resolved when reusing