At certain window sizes, the topic progress indicator “dances” up and down, which at best causes scroll bars to appear and disappear, and at worst causes the entire composer to jump up and down.
In Chrome dev tools, I can see the element alternating between class=“ember-view” and “ember-view-docked” which have different CSS.
To reproduce the bug, go to any topic and scroll so that the footer buttons (Invite, Bookmark etc.) are just at the bottom of the window, then shrink the window to <903px wide.
You can do the same thing while composing a message, if you scroll the footer buttons so they’re just above the composer.
I can reproduce the bug in Chrome43 and IE11