I normally use Alt/Enter to switch between minimazed and full screen, since yesterday, it works no more as before, when I hit the combination, the screen switches from full screen (borderless in fact), to a bigger screen which exceeds the borders. Alt/Tab doesnt work either, and also selecting window, or full screen at the MSFS graphic options. So I cannot access to other aplications while MSFS is on screen.I wonder what had happened.

I have similar but slightly different issues. When I change from full to windowed the screen goes black (because it cannot display properly). If I want windowed I have to close MSFS  in windowed mode. And on the next reboot full will not work (black screen). Or close in full screen if I want that to use full screen on the next MSFS bootup. I think it has to do with how MSFS interacts with the graphics card. Maybe the older dlss and dlssg files would work better than the newer ones I am using. I'm using 3.7.1.10 versions I think. Properties of the files will show their version.

I'll try that

I think my problem began when I fiddled around with apps as Lossless Scale FG, Msi Afterburner and RTSS,specially LSFG with which I had strange behaviours when switching screens . So I uninstalled all of them

Yes, your symptoms are something I noticed , you can't use the alt+enter trick with lossless app.

 

Bingo Fielder, thanks, your advice was succesfull. By hitting Alt/enter I switched to that oversized screen, but then as the MSFS inferior menu bar was no more visible, I had to kill MSFS via the task bar, and at the next boot up, all was OK,