Fix z-index for diff file with active dropdown in Rapid Diffs

Stanislav Lashmanovrequested to merge
slashmanov/rapid-diffs-fix-file-z-index into master

What does this MR do and why?

This MR fixes a regression after !191782 (merged).

It changed the root element of the file header with position: sticky from .rd-diff-file-header to .rd-diff-file-header-sticky. But z-index wasn't moved to the new element, which caused for this bug to reappear.

Screenshots or screen recordings

Before After
image image

How to set up and validate locally

  1. Enable rapid_diffs and rapid_diffs_on_mr_show feature flags
  2. Open any merge request with at least 2 files
  3. Select 'Changes' tab
  4. Add ?rapid_diffs=true to the URL, follow it
  5. Collapse the first file
  6. Open menu on the collapsed file
  7. Observe dropdown being on top of another file, not covered by anything

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