Thumb blocks the right side of a maximized window

You mean widgets below the "thumb"? (we call thumb the movable widget that is used to scroll)
In this case, almost the text could be below, and we might decide to add a timeout in order to make it disappear.

I agree with the importance. I don't think I have ever seen an app actually put a small widget on the far right within a scrolled container. The one case that springs to mind is the Eject buttons in Nautilus's side pane, but that pane is on the left so this will never reasonably happen there. Still, it feels like something that isn't quite perfect :)

I would add a timeout... asking Mark

Yes, the design called for a timeout, so a user can pause, let the thumb evaporate, then do whatever they want behind it.

I'm not sure the behavior is perfected yet. Right now I see two problems with it:

1) The evaporation stops if the mouse arrow is moved by even one pixel
2) The evaporation takes too long (this is more of an opinion)

I would suggest the following changes:

1) Increase the speed of the evaporation
2) To make up for the shorter evaporation time, increase the time until evaporation
3) Continue evaporation as long as the mouse arrow remains over the thumb