Finish and ship unified document zoom (take over #518) — v3000.0.8 #529

Goal

Ship unified document zoom as a v3000.0.8 headline feature by taking over draft PR #518 (@leevi2010-cursor, "Unify editor and preview zoom") and finishing it maintainer-side. The design is specified in #470 (original request #335). The architecture in #518 was reviewed and judged sound — the remaining work is wiring bugs, a keybinding decision, and test hygiene, not a redesign.

This issue is the execution plan. Implementation has not started.

Branch plan — de-entangle from #519 first

#518 is 9 zoom commits off main plus a duplicated tip commit (106aff0) that is byte-identical to PR #519's sole commit ("Remember editor pane visibility"). That change (overloading the user-facing editorStartInPreviewMode preference) has an unresolved design concern and is being handled separately on #519.

• Branch our own work off origin/main from Unify editor and preview zoom #518's zoom commits, dropping the 106aff0 tip commit (e.g. rebase omitting it). The Remember editor pane visibility #519 content — both the -toggleEditorPane: change and its MPPaneToggleTests.m test — is a fully isolated tip commit with zero interweaving into the zoom code, so this is a clean drop.
• Remember editor pane visibility #519 remains its own open PR; its design question is resolved there, not here.

Fixes

1. Toolbar zoom dropdown does nothing (functional blocker)

MPToolbarController.m's new toolbarItemDocumentZoomPopUpWithIdentifier:label: sets added.target = self.document at construction time, when the document outlet is still nil, so clicking a preset is a silent no-op.

• Route the popup through the file's existing deferred-dispatch idiom (standaloneToolbarItemClicked: / dropdownMenuItemClicked:): set target = self and resolve self.document lazily at click time. selectDocumentZoom: already accepts both NSPopUpButton and NSMenuItem senders and reads the level from representedObject, so forwarding is a few lines.

2. Format-shortcut relocation (decided)

Zoom takes ⌘+ / ⌘− / ⌘0, which collide with three Format items. #518 currently just deletes those bindings. Decision, grounded in cross-editor convention and MacDown's own free keys:

• Apply the three xib changes in MainMenu.xib; add the Zoom In / Zoom Out / Actual Size items on ⌘+ / ⌘− / ⌘0.
• Verify no new collisions (⌘⇧X and ⌘⌥0 confirmed free against the current xib).

3. Reconcile the two contradictory zoom test suites

MPPreviewZoomTests.m (preset-snapping on a shared preference) matches the shipped implementation and is authoritative. MPZoomTests.m mixes ~3 tests that encode a false flat-±0.1 stepping model with a majority of model-independent tests (menu validation, zoomed-font/tab-stop rendering, previewScale).

• Delete the ~3 wrong-model stepping tests; keep the rendering/validation tests (consider renaming the file to signal it covers rendering, not stepping).
• Confirm CI green with the retained suite; test count ≥ baseline.

4. Cross-window zoom sharing test

The mechanism works for real open windows (documents observe NSUserDefaultsDidChangeNotification → applyCurrentZoom), but the observer is registered in windowControllerDidLoadNib:, so headless tests can't see propagation and none currently does.

• Recommended: small refactor extracting the zoom-observer registration out of the nib-load path so a headless test can assert doc A's zoom change propagates to doc B. This proves the headline behavior.
• Fallback if the refactor is riskier than it looks: a thin shared-preference test plus manual cross-window verification before the RC. Decision to confirm when implementation starts.

5. Quick include: clamp bypass

resetZoom: / selectDocumentZoom: write documentZoomLevel directly, bypassing the clamp in setZoomMultiplier:. Safe today only because presets match the bounds.

• Route these through the clamping setter so bounds enforcement lives in one place.

Release / merge notes

• Migration: Unify editor and preview zoom #518 bumps preference migration to v6 (sets documentZoomLevel = 1.0). Purely additive, touches no other keys — low RC risk.
• Merge conflict: expect one mechanical MacDown 3000.xcodeproj/project.pbxproj conflict against the Internal anchor links are not clickable in exported PDF (inherited from upstream MacDown) #504 work already on the milestone branch (both add to different subsections). No source-file conflicts.
• No Sparkle 2 overlap: Unify editor and preview zoom #518 touches none of the Sparkle files; the two headline features are independent.

Acceptance criteria

• Debug build succeeds; full test suite green; test count ≥ baseline.
• Toolbar zoom dropdown actually re-zooms the document when a preset is clicked.
• ⌘+ / ⌘− / ⌘0 drive zoom; Strikethrough (⌘⇧X) and Paragraph (⌘⌥0) work at their new bindings; Highlight's drop is intentional and documented.
• Zoom level is shared across open document windows and persists across launches.
• Migration v6 applies cleanly for existing users.
• Rule-of-Two review passed before merge.

Follow-ups (not this issue)

• Rebind Highlight to a sensible shortcut (its own issue).
• Optional future parity: move headings from ⌘1–⌘6 to ⌘⌥1–⌘⌥6 (Google Docs scheme), which would let Paragraph ⌘⌥0 sit with its siblings. Out of scope for the zoom PR.
• Resolve the Remember editor pane visibility #519 preference-overloading design separately.

Related to #470, #335, #518, #519, #504.