R1:27bdc322fc11

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

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