R1:8c1a1312e8c7

Improve column device behaviors

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…