Description

When testing scenarios involving large note threads (e.g., threads with many state changes or a high number of replies), expanding a note thread causes the replies to extend below the visible viewport in the sidebar.

Scrolling with the mouse wheel works as expected — the user can scroll through the long list of replies without issues.

However, when the user attempts to scroll using the sidebar scroll bar by clicking and dragging it:

• The expanded note thread collapses immediately.
• All visible replies disappear.
• The user is unable to view the thread replies unless the thread is re-expanded.
• This occurs consistently whenever the scroll bar is dragged.

This makes it impossible to review long comment threads using traditional scroll-bar interaction.

Step-by-step reproduction instructions

1. Open a post with the collaboration sidebar enabled.
2. Locate or create a note thread with a large number of replies (or repeatedly toggle Resolve/Reopen to generate long threads).
3. Expand the thread to reveal all replies.
4. Attempt to scroll using the mouse wheel → replies remain visible.
5. Attempt to scroll using the scroll bar drag → thread collapses immediately.

Expected Behavior

• Expanded threads should remain open while scrolling, regardless of the scrolling method.
• Dragging the scroll bar should behave the same as mouse-wheel scrolling.
• Users should be able to scroll through long reply threads without the thread collapsing.

Screenshots, screen recording, code snippet

Environment info

• WordPress version (6.9-RC2)
• Gutenberg Version 22.1.2
• MacBook Pro
• Chrome Version 142.0.7444.176

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure