I don't know if the following is relevant... My emacs version is: 27.0.50, compiled from sources. I use emacs in full screen mode namely with: (toggle-frame-fullscreen) in my dot-emacs. In fullscreen mode, I don't have access to the panel, where my Spectacle widget is. And my purpose is to take a screenshot of emacs. Also Spectacle is configured so that it doesn't show any dialogue window, but instead goes directly to "select an area and then press enter; and then save the image silently; and go away". So I go on "konsole" (so I have access to the panel), then click on Spectacle, then do "Alt-Tab".
First issue, I should be over emacs window, but it still shows me konsole window. (I've got a thumbnails column of all my windows when doing Alt-Tab, so I know I should be over emacs, but it doesn't show). Then it tells me that I can press Escape, to leave. But after doing so, it seems I have no mean to do actually anything anymore: plasma session is inaccessible (Alt-tab, or moving the mouse the the upper- left corner have no effect anymore; pressing "escape", or "enter" neither), and I can't leave Spectacle. Only solution I've found: go in another tty and do a killall spectacle. And when I come back, emacs which was originally "divided in two", isn't anymore, so it's been tampered during the operation. I don't know if that would be reproducible with a debian's version of emacs. Thanks, Chris