R1:415de4ce375a

Repaint filetree more consistently for mobile/device views

Summary:
Ref T13189. See <https://discourse.phabricator-community.org/t/diffusion-differential-mobile-layout-broken-when-enabling-file-tree/1751>.

We currently call a nonexistent `resetdrag()` which does nothing. Some sequences of interactions can result in a blank left column in mobile/device widths.

Repaint the filetree away more consistently on device change.

Test Plan: Viewed a revision, toggled filetree off + on, resized to narrow width.