The screen refresh of the secondary monitor on my setup, is something that usually clears itself up after a few moments,
Unfortunately, that is not the case here. The affected areas will stay that way until/unless I take a window and "swipe" across it, forcing a refresh.
I do think you should try changing over the 'Primary Monitor' to one of your other screens, to see what if any difference it makes. It can and does affect the refresh of the backgrounds on my system, though not always! - I have assumed that this is a video card/driver problem, more than a MA3 bug.
I went ahead and tried it here but it makes no difference. Regardless of which monitor is set to primary, when MA3 switches to windowed mode it shows up on the primary monitor whose background is properly refreshed while the other two monitor backgrounds are not.
The large drop in FPS for a window covering a part of more than one monitor has been present since MA3 went to multi screens.
In reviewing my earlier post, I didn't make it as clear as I should have. The point I was trying to make was that prior to the 7G version, the frame rates stayed up in the 30's as long as the window did not touch more than one screen at a time. It didn't matter whether the window was on screen #1, 2 or 3. So long as it did not span more than one screen, the frame rates stayed up in the 30's. Now with 7G, it only exhibits that behavior on the primary monitor on my system. If the window is on either of the other screens, even if it is not spanning multiple screens, the frame rate drops to and stays at ~5-6fps. Provided that this can be duplicated on other systems, perhaps that info along with knowing just what parts of the coding were changed to fix the "frozen fish" bug will help narrow down the cause of the low frame rates when spanning screens with a window.