The reason I was modifying the frame here by one padding's worth was to account for the "doubling of padding" between windows (i.e. between the main screen and secondary pane windows).  What happens is if they both apply padding, you end up with a 10px padding between the windows and edges of the screen, and a 20px padding between windows which looks a little off.  To handle that, I had the screen have its fixed padding around the edges, then each window only takes care of the non frame edge padding.

I will see if I can move this logic into assignFrame, but it may require passing knowledge about whether it is a main pane window or a secondary pane window (I'm worried about using this way of thinking because of layouts that do not have a notion of main or secondary panes).