Fix filetree drag nav CSS

Summary: Fixes T11630. Not sure what the max-width fixes, but I don't see anything off on various mobile, desktop.

Test Plan: Enable filetree in differential, drag navigation all over, see normal width calculations.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11630

Differential Revision: https://secure.phabricator.com/D17591