Avoid overflow in ChatRoom's mainContainer.
Fix some layout issues with chat.
Prevent overflow in topPane.

The layout functionality of ContentPane was kicking in and making the
TabContainer too large.
Add border-style declarations to fix margin box.

Without border-style (i.e. with border-style: none) webkit (and by
extension dojo) reports the margin box of elements ignoring the border,
even when when border-image is set.