There are a few issues with the media manager at the lower media query breakpoint.
* Check marks in checkboxes overflow their containers: http://i.imgur.com/qcQ3qBv.png (You can see this on the second step when adding a gallery).
* The layout is generally a bit funky. The entire window scrolls instead of just the attachments container.
* There is a point between the narrowest media query and the next one where the sidebar overlaps the main content: http://i.imgur.com/VHAeuim.png There's also a dead spot over a right hand portion of the second row of images, making it impossible to select them.