Built with AI, in the open

The Oracle performed a deep review of EDITOR_UI_REFERENCE.md and caught 15+ bugs — off-by-one errors in viewport calculations, division-by-zero edge cases in scrollbar math, and mismatched struct field semantics — before any of them became code bugs.

Undo/redo was completely broken on macOS. Pressing Cmd+Z inserted the letter 'z' into the document. Root cause: the key handler only checked control_key(), not super_key() (macOS Command key). A one-line fix.

Implementing multi-pane editing required a shared-documents/independent-editors architecture. Documents live in a HashMap, multiple editors can view the same document with independent cursors and viewports. Migration helpers like single_document() enabled phased refactoring.

Arrow keys and all movement operations only affected the primary cursor — secondary cursors were frozen. Every movement handler assumed cursor_mut() returned the only cursor, but it was actually cursors[0]. Fixed by creating per-cursor primitives and all-cursor wrappers.

A complete spreadsheet UI for CSV/TSV files built across 6 sessions: RFC 4180 parsing, grid rendering with column headers, arrow key navigation, Tab/Shift+Tab cell movement, inline cell editing with proper CSV escaping, and Enter to confirm + move down.

Five separate text editing implementations (main editor, command palette, go-to-line, find/replace, CSV cells) with duplicated logic. Solved by creating a unified EditableState<B: TextBuffer> abstraction that shares editing logic across all contexts. All modals gained full cursor navigation, selection, and clipboard support.

Multi-split view caused ~7 FPS with 100% CPU even when idle. Root cause: ControlFlow::Poll was spinning the event loop constantly. Fixed by switching to ControlFlow::WaitUntil — idle CPU dropped from 100% to 0%, FPS jumped to 60.

Built a full docked panel system with webview-based markdown/HTML preview across 9 sessions. DockPanel abstraction with resizable panels, WKWebView on macOS with custom token:// protocol for local resources, and live preview updates on tab switch.

Mouse handling was ad-hoc if/else chains checking pixel coordinates. Replaced with a priority-ordered HitTarget enum and hit-test system. Each target knows its bounds, first match wins — clean, extensible, and testable.

Clicking below the midpoint of the editor caused unwanted scrolling. A Mermaid diagram revealed the root cause: new editors initialized with visible_lines=25 (hardcoded default) but the actual viewport showed ~45 lines. The model thought line 30 was "off-screen" and scrolled to compensate. Fixed by syncing viewport dimensions on tab/split creation and adding no-padding scroll for clicks.

Every keystroke spawned a new thread for syntax highlighting debouncing, causing visible highlight flashing. Fixed with two changes: replaced thread-per-debounce with event-loop deadline timers (no new threads per keystroke), and added immediate highlight shifting that adjusts existing highlights by the edit delta before the background parse completes.

No way to quickly switch between recently opened files. Built a persistent MRU list with Cmd+E modal across 5 phases: JSON persistence for up to 50 entries, fuzzy filtering with file type icons and "time ago" timestamps, and automatic tracking from all file-open methods (CLI, dialog, sidebar, drag-and-drop).

Built a tree-sitter-based code outline panel supporting 10 languages (Rust, TypeScript, JavaScript, Python, Go, Java, PHP, C/C++, Markdown, YAML). Uses range-containment algorithm for code and level-based hierarchy for Markdown. Runs on the syntax worker thread alongside highlighting. Critical bug: dock clicks fell through to the editor because hit-test priority didn't include docks.