In the new 5.1 update, Moving the mouse slowly keeps the mouse in the center of the window but moving the mouse quickly or at a fast speed makes the cursor leave the box. Mouse is clicking through desktop once detached from the window when using games or apps in steamvr.
Tried the new 5.1.1.3 alpha, and the mouse capture window is better but has major flaw.
Moving the mouse too quickly upwards to the top border and clicking will pass-through the mouse click and lose lock. Bottom, Right, and Left borders do not show this behavior. Just the top Border.