Resizable items below the "Drag to resize" handle are not resizing properly in Firefox

[remcokalf]

Description

Resizable items below "Drag to resize" handle are not resizing properly. Dragging their handle results in stuttering/glitching. This happens for example with an ACF text area field.
The issue happens in Firefox only it seems. I don't see it in Chrome.

This issue happens will every resizable item. Not only textareas, also items that have e.g. resize: vertical set in CSS.
Resizable items outside of this area are resizing just fine, like the post title textarea in code view.

See below screenshot of and example with an ACF text area field.

Not entirely sure if this is a Gutenberg issue, or if I should add it in trac.

Step-by-step reproduction instructions

• Install ACF or another custom field plugin, eg. Custom Field Suite
• Add a resizable field, e.g. a textarea field
• Open a post/page edit screen in Firefox
• Try to resize the field

Screenshots, screen recording, code snippet

In Firefox:

Environment info

• Happens in Firefox only
• Happens in WP 6.7 and 6.7.1
• Does not happen in 6.6.1 (only other version I could test on).

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

[hanneslsm]

Can confirm.
Tested on MacOS and Firefox 132.0.2 (aarch64)