Resize the Differential keyboard focus reticle when the window is resized

Summary: Fixes T12632. Ref T12634. Currently, the keyboard focus reticle does not redraw properly after a window resize.

Test Plan:

• Used "n" to select a block.
• Resized the window.
• Saw the reticle also resize properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12634, T12632

Differential Revision: https://secure.phabricator.com/D17901