Scroll position is lost or jumps when switching between Editor and Preview

[threatner]

What happened?

Two related issues when switching between Editor (⌘1) and Preview (⌘2):

1. Editor → Preview:
   Scrolling to the middle in Editor opens Preview at the top.

2. Preview → Editor:
   Scrolling in Preview and switching back makes the Editor jump to the bottom.

Expected:
Scroll position should be preserved in both directions (as intended by ScrollBridge in PositionSync.swift).

Root cause:

computeScrollFraction uses:

clipView.bounds.origin.y / maxScroll

With flipped NSTextView coordinates, this becomes negative when scrolled:

ScrollBridge.set[...] = -0.5863

Preview assumes a 0–1 positive fraction and calls:

scrollTo(0, fraction × maxScroll)

So:

• Negative fraction → clamped to top (Editor → Preview)
• Positive fraction from preview → misapplied by editor → clamps to bottom (Preview → Editor)

This is a coordinate convention mismatch between editor and preview.

Steps to reproduce

Symptom 1 (Editor → Preview):

1. Open any .md file long enough to need scrolling.
2. In Editor mode (⌘1), scroll to roughly the middle.
3. Press ⌘2 to switch to Preview.
4. Preview opens at the top instead of the middle.

Symptom 2 (Preview → Editor):

1. From the previous state, scroll the preview to any position.
2. Press ⌘1 to switch back to Editor.
3. Editor jumps to the bottom instead of matching the preview position.

App version

1.13.0

macOS version

26.4

Screenshots

Screen.Recording.2026-04-10.at.12.08.17.AM.mov

Problematic file

No response

[threatner]

Happy to put up a fix PR if the diagnosis sounds right to you @Shpigford.