* Click and hold a scrollbar
* Drag the cursor onto another surface
* While still holding the original button, press and release another
cursor button
* Things get weird
There's two ways to fix this. Either cancel the seat operation and do
the other click, or continue the seat operation and ignore the other
click. I opted for the latter (ignoring the click) because it's easier
to implement, and I suspect a second click during a seat operation is
probably unintentional anyway.