Boost resizes windows multiple times, for example during page load, when navigation is expanded/collapsed. This should trigger resize events.

Replication steps
Create a few assignments with a name long enough to get a gradebook extending horizontally
Go to the gradebook and open the drawer
Scroll to the right to get the students' names to float
Scroll back to the left so that the names don't float
Close the drawer
Scroll to the right
The names do not float any more

Replication instructions reported by Fred.
Also other areas that this effects - MDL-57090 and MDL-56779