Editor and Preview trying to sync when disabled

[gregwillits]

I opened the app with Sync Panes set to true, opened a file, started editing/scrolling etc.

Decided I didn't want the two panes sync'd. Opened Settings, disabled the checkbox. I could scroll the panes independently as expected.

However, when I started to type in the Editor, the Preview pane would auto scroll to try to sync. I could still scroll the Preview pane independently, but each time I started to type in the editor, the scroll position changed.

If I quit and reopened everything (now with Sync Panes disabled to start with), then that behavior stopped (as desired) — typing did not cause scrolling in Preview, and the two pnes were correctly completely independent.

[schuyler]

Requirements gathered

Thanks for the detailed report — the "restart fixes it" detail was the key clue that this is a mid-session state problem, not an initialization one.

After investigating the scroll-sync subsystem (MPDocument.m, ownership model from #342/#366/#379), here is the agreed scope:

Core fix — disabling Sync Panes mid-session

• Turning Sync Panes off while a document is open must make the panes fully independent immediately, in both directions. In particular, typing in the editor must not move the Preview — matching the behavior you get when launching with sync already off.
• Current scroll positions are preserved at the moment of disabling (no jump).

Symmetric fix — enabling Sync Panes mid-session

• Turning Sync Panes on while a document is open will re-sync immediately (Editor authoritative: the Preview scrolls to match the editor's current position), then continue syncing normally.

Audit

• Every scroll-sync trigger (typing/DOM-replacement, editor/preview bounds changes, live-scroll start/end, full-reload completion, resize/fullscreen, split-divider reveal, checkbox toggle) will be reviewed to ensure it honors the current value of the sync preference and that no stale ownership/cached state survives a mid-session toggle in either direction.

Testing

• Test-driven, extending MacDownTests/MPScrollSyncTests.m (which drives a real MPDocument). New tests will cover the disable-mid-session regression, the typing-while-disabled case, and the enable-mid-session re-sync.

Implementation will follow on a claude/ branch; PR to come.

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Generated by Claude Code

[schuyler]

👋 @gregwillits — A fix for this is available for testing in MacDown 3000.0.7-rc.1.

If you have a moment, please download it, confirm the issue is resolved, and
reply here — your confirmation is what graduates the fix into the next stable
release (currently planned for 2026-07-10).

📦 Download: https://github.com/schuyler/macdown3000/releases/tag/v3000.0.7-rc.1

First launch: if macOS says the app is from an unidentified developer,
right-click the app and choose Open, then confirm. (This is a signed,
notarized pre-release build.)

No rush — if we don't hear back, the fix ships anyway. Thanks for reporting it!