On a folded Duo, a PR's Files, helpers, and Work on PR drew over the
Threads and PRs tabs at the bottom of the rail. The workspace tab bar
and each split column's toolbar belong to different containers, and
UIKit stacks neither against the other in the vertical bar. The folded
list's actions were covered the same way.

- Workspace columns move their toolbar items into their navigation
  items when the vertical bar is present, separated from the existing
  items by a fixed space. The expanded sidebar keeps its horizontal
  toolbar.
- Modal stacks have no tab bar in their vertical bar and keep real
  toolbar groups.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>