Root cause is unclear. I've removed gr-fixed-panel from element
entirely, and that had no impact on gr-selection-action-box
positioning.
My strongest theory is that some node inside diff added some style that
affected positioning calculation (position, display, etc)
To ensure this would not repeat, gr-selection-action-box is now created
as a first child of gr-diff-highlight, which has position:relative
applied.
Bug: Issue 6567
Change-Id: I73423d62ce4eb7bd31e74fa2e561b2e850ba95be
1c8c053b07