I just had a Win 10 update this AM, and it may be a coincidence, but Alt-Tab functioning changed on me. I play Morrowind GOTY, with MCP and MGE XE at 3440x1400 borderless fullscreen beautifully. Today, when I Alt-Tab and come back to Morrowind it opens a full screen window with border that is all black and I can't get rid of it. There is no cursor, and I can't Alt-Tab out of it. All thoughts welcome!

Are you sure that alt-tab worked before? What you describe is a common experience when alt-tabbing, and in no way limited to Win 10 or 11. Morrowind was simply never designed to support it.

It's been working. I hadn't realized it was unusual. Could GOTY from GOG be different than the CD OG in that regard? It still works fine on the Construction Set. Thanks!

I don't think so. If you're interested in more information, search the internet for "morrowind alt-tab" or so.

There is a recommendation to edit the morrowind.ini: Change
Background Keyboard=0 to Background Keyboard=1.

I myself haven't tried it.

Thanks for the pointers. It seems that MGE XE was letting it work. I'm not sure why it changed on me, but in MGE XE I changed out of and back to borderless window and alt-tab is working fine again.