File Tree Browser Ignores Layout Width User Preference

Summary

The new file tree browser ignores the layout width user preference, always forcing full width.

This makes auto-scroll (middle mouse click + drag to pan/scroll) difficult to use because:

1. Middle mouse clicking on the file browser scrolls the file browser pane (this is fine).
2. Middle mouse clicking on the file preview only allows horizontal scroll if x-overflow is triggered (cannot do vertical scroll).
3. There is very little whitespace available to middle mouse click for guaranteed full page vertical scroll (only the vertical margin flanking the file browser and file preview panes).

Steps to reproduce

1. Set User Settings → Preferences → Behavior → Layout width to Fixed.
2. View a repository's files.
   • Preferably a long plain text file that triggers x-overflow. Auto-scroll on the preview works fine with rendered files like rendered Markdown or files that don't trigger x-overflow.

Note that auto-scroll isn't supported by some browsers on all platforms (e.g. Firefox enables it on all platforms by default, Chromium-based browsers only enable it on Windows by default).

Example Project

https://gitlab.com/gitlab-org/gitlab

https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab-ci.yml?ref_type=heads

GitLab Enterprise Edition 18.8.0-pre.

What is the current bug behavior?

The new file tree browser forces a full width.

What is the expected correct behavior?

The new file tree browser should support a fixed layout width. See design recommendation #585779 (comment 3006602487).