If she leaves her machine she uses fast user switching to go to the login window. On return there are large while rectangles positioned where some former apps used to be. They behave like the X11 root window; her X11 root window context menus function in these areas.

I too find that the white rectangles do not appear until I switch to the main "Mac" display and back again. Thereafter, they appear in both. This suggests a bug is not in the code that removes windows when going from X to Mac display, but in the code that restores them when switching the other way. Perhaps its test for which windows to revive is faulty. If I understand correctly, the function is RootlessShowAllWindows() in rootlessWindows.c. It restores all windows that are "realized", but I'm not sure how "realized" relates to "mapped". -- Richard -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.