The review request box historically was built with floats and

JavaScript-based layout. While this generally worked, we had an issue

with the Testing Done box in 6.0 where its height could get capped to

the lower bounds of the Details pane.
A JavaScript workaround wasn't the solution. Moving to modern CSS-based

layout methods was the way forward.
The box is now built using our standard CSS Component style, and uses

CSS Grid and Flexbox to manage the layout. This is faster, future-proof,

and allows us to eliminate a lot of JavaScript.
The old CSS was a bit of a mess, without a clear structure. We now have

a specific container for the field panes, and a container for each pane.

These are set up as grid areas, ensuring we can lay them out exactly as

we want.
There are no differences in layout, with the exception that the details

pane now has a bit more breathing room, which is easier to spare on

modern displays, and more necessary with the recent fixes to the display

of inline editors.
The review request boxes have also been updated to have ARIA labels.

These are fairly generic right now, but should help with navigation

nonetheless.

Tested the review request page, diff viewer, review UIs for images,

text files, and PDFs, making sure there were no regressions. These were

checked on desktop and mobile.