R1:ac570fd4bc4e

When you make the file tree huge, scroll to the right, and then toggle it, stop it from growing

Summary: Depends on D19346. Ref PHI568. I love Javascript.

Test Plan:
- Viewed a revision.
- Dragged file tree view really wide.
- Scrolled document to the right.
- Toggled file tree off and on by pressing "f" twice.
- Before patch: file tree grew wider and wider after it was toggled.
- After patch: file tree stayed the same size after it was toggled.
- Dragged to various widths and reloaded…