Reduce cleverness of tablet breakpoint adjustment

Summary:
Fixes T7713. 2 much math X.X

Specifically, we could end up with some nonsense widths here (smaller than the phone breakpoint) with an initially-hidden column.

Test Plan: Hit tablet breakpoint with column open and closed.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7713

Differential Revision: https://secure.phabricator.com/D12225