When working with 2 screens (identical size/type/resolution), if I right click over the score a menu opens up at the place of my mouse pointer. When I try to choose a subcategory the menu opens up a millisecond at the correct place to the right but then jumps all the way over to the left screen. Some of the categories will open a correctly placed popupwindow the first time I hover my mouse over it (but not the second time- then it will jump to the left), and it will stay there, but going down another level will make the new popupwindow appear in the left screen. When I try to moove my mouse pointer over to the left screen, the popup closes. The only way to access my chosen menu-item using my mouse, is to move the score-window over to my left screen and repeat the process there.
There are a variety of interface quirks when working with two or more displays, due to bugs in the Qt framework that the developers use (to provide Windows and Mac versions together).
Keeping everything within one display seems to minimize them.