Fix diff file options clipped by merge request sticky header

What does this MR do and why?

Fix diff file options clipped by merge request sticky header

Screenshots or screen recordings

Before	After
CleanShot_2025-12-13_at_00.18.48_2x	CleanShot_2025-12-13_at_00.18.19_2x

How to set up and validate locally

1. Go to any merge request with changes
2. Open Changes tab
3. Shrink viewport to 200 pixels height
4. Open diff file options menu for the first file
5. Observe all options to be selectable