[WIP] Make it possible to switch between files without scrolling to the top

Review Request #10362 — Created Dec. 14, 2018 and discarded — Latest diff uploaded


Review Board


When reviewing it can be useful to jump back and forth between specific
files or parts of the patch. Right now this requires scrolling all the
way to the top of the page and clicking the desired file, or remembering
where it is (which can be difficult in a large diff).

To improve this experience, the file list will dock itself as a
"banner widget" into the unified review banner. As a "banner widget",
the file list will work as a drop down showing the current file and
expanding to jump to other files.