Put the keyboard focus reticle back on top of diffs
Summary:
Fixes T10954. This got hidden underneath things at some point.
Use `pointer-events: none` to make the mouse ignore the element so that hover/select/edit/click still work "through" the element.
Design could probably be improved here, maybe I'll make it more-visible when you press {key n} and then have it fade quickly so it kind of gets out of your way once you find the block you want to read.
Test Plan: {F4921746}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10954
Differential Revision: https://secure.phabricator.com/D17784