It is a windows 'thing' and its origins go back to Windows 3.1 it is an issue with the way windows draws a box around a program and that arose when MS basically copied IBM's X-Windows to develop GUI Windows displays. I have never found a fix but it is unpredictable. The only way I can fix it is to use TASK MANAGER and force a swap to the FSX program which should show as running, sometime this will not work either so use TASK MANAGER to shut it down, log off and restart the program. This is a legacy issue which I think will carry forward forever it may be an addressing issue vs a vis the display driver but I cannot find the answer but for some reason FSX will generate this problem not frequently but now and then.
Sorry cannot be any more help other than the process I use above which works, sometimes yes, sometimes no. Guess that's what you get when you go from 8 to 16 to 32 to 64 bit systems and basically underneath is now a hybrid DOS/UNIX platform overrun with JAVA, HTML etc. you get the picture. This is also the reason I am remaining with Windows 7, I would ditch it tomorrow for a LINUX/UNIX platform except I can't due to FSX and another legacy program I also run (LOTUS). I only use Windows as an operating system and everything else is not MS (and deleted from the system permanently), and I mean everything, browser, office programs, video-media etc. etc., life is a lot more peaceful this way. You really get tired of being the home systems adminstrator and help desk with MS.
If your running a laptop then your a bit stuck as they always ship these with Windows as that is what business uses. I do not have a laptop anymore but ditched it when smart phones arrive, just a desktop for FSX and media everything else in this house happens on android thingys