Maximize a Thunar window that has enough files/dirs in it to require vertical scrolling. Move mouse all the way to the right against the screen where you should be able to grab the scrollbar. BUG: Cannot grab the scrollbar with the mouse. Need to move mouse in a little before you can do that. -- In general, though, I ask the question: Even when not maximized, do we really want a scrollbar-side border of the Thunar contents? Seems like the way Firefox does it is the way to go: no border outside the scrollbar. But obviously there is more structure within the pane than FF, so perhaps (?) internal borders or column divisions would be useful.
I'm marking this a duplicate of #7496 which features a mockup of the desired look. Pretty sure this will land in the next stable version of Thunar. *** This bug has been marked as a duplicate of bug 7496 ***