Summary:
Fixes T7062. When the column is open, we only want to consider the screen width which is avilable for content when computing responsive breakpoints.
Specificially, if you have a 1000px wide browser window (normally "desktop") but the column is open (300px) so you only have 700px free for content (normally "tablet"), we should drop to the tablet breakpoint. This lets you have a narrow column of "tablet" content next to the chat column, instead of a really squished column of "desktop" contnet.
This also means the chat column can't directly use JX.Device to hide itself.
Test Plan: Resized screen with column open, saw content go from Desktop + Column -> Tablet + Column -> Tablet -> Mobile.
Reviewers: btrahan, chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T7062
Differential Revision: https://secure.phabricator.com/D12189
8c1a1312e8