Redesign the interface around native macOS chrome and a real design system #14

Summary

Replace the hand-built interface with native macOS chrome and move every visual decision out of the views into a design system. Landed in #13.

Motivation

The window had the shape of an application without the finish of one. Two defects drove most of it:

• ThemeManager.isDark was background.red < background.blue, which is false for any neutral background — #1e1e1e is 30 < 30. Dark and High Contrast therefore forced .preferredColorScheme(.light), so every system-drawn control rendered light against a dark canvas. It read as a styling problem and was a comparison operator.
• The tab strip swapped the close button for the dirty dot, so modified tabs could not be closed without saving first.

Beyond those, the chrome was 13 hardcoded .system(size:) calls with no semantic styles, 7 off-grid padding values, zero corner radii, fixed 28pt/20pt bars, two competing empty states in different capitalisations, a hand-rolled sidebar header that truncated No Folder Opened to No Fold...Opened, and Theme.keyword doing five unrelated jobs — app tint, tab underline, folder icon, terminal prompt and terminal errors.

Proposed approach

Modules touched: CodeEditorThemes (tokens), CodeEditorCore (metrics, extracted logic), CodeEditorUI (views), CodeEditorApp (toolbar, commands).

1. Theme declares colorScheme per palette, plus Chrome and Semantic groups. Chrome never borrows a syntax token.
2. Metrics — 4pt grid, radii by surface, chrome heights — in CodeEditorCore, Foundation-only, grid enforced by a test.
3. Typography — semantic styles so chrome scales with system text size; code stays fixed-size SF Mono.
4. Materials: stop overriding the sidebar's own vibrancy.
5. Unified toolbar replacing the sidebar header; tabs that merge into the canvas; one adaptive empty state.
6. Selection-driven tree activation, the Xcode navigator rule.
7. ⌘F via CodeEdit's own panel, ⌘P quick open, drag-resized terminal, session restore.
8. FileOutline, QuickOpenMatcher, QuickOpenIndex, SessionStore extracted to Core and tested.

Ordering matters: finish before features. The find panel is upstream AppKit, so dropping it into an unfinished shell makes the gap more obvious, not less.

Trap for whoever picks this up

• A row .onTapGesture inside a List(selection:) never fires — the List consumes the click. Drive row behaviour from the selection change. See MEMORY.md docs: project documentation set plus AGENTS.md #12.
• NavigationSplitView columns are sized in exactly one place. See MEMORY.md feat(ui): redesign the interface around native macOS chrome #13.
• Materials follow preferredColorScheme, which the palette drives. Vibrancy and "theme drives everything" are compatible because of that, but High Contrast must set usesMaterials = false.

Acceptance criteria

• isDark no longer derived from colour arithmetic; regression test covers all three palettes
• Dirty tabs are closeable without saving
• No .system(size:) in chrome; no padding off the 4pt grid
• One empty state, one primary action per surface
• Sidebar reveals and emphasises the active document
• ⌘F, ⌘P, resizable terminal, session restore
• swift build passes
• swift test passes — 68 tests, up from 35
• Visual review by a human. Spacing, materials, contrast and the tab/canvas merge were never confirmed by eye; the implementing session could not read screenshots.

Not done, deliberately

LSP transport, PTY terminal, external-change detection, Finder file associations (needs CFBundleDocumentTypes), new dependencies, a new app icon.