See dirkts hint which turned out to be very useful: Using "xlsclients -la", I found a list of X apps, including those running in the background. I started killing them; some of the process terminations made my Gnome session break down, but I eventually found that shutting down the skypeforlinux process made CTRL-ALT-SHIFT-d work for me. It turns out that in Skype, an extra "Debug" menu item appears, if you press CTRL-ALT-SHIFT-d. It seems this behaviour cannot be disabled in Skype.

See dirkt's hint which turned out to be very useful:

I don't know any better way than killing X clients one after the other, possibly using xlsclients etc. to guide you, and testing to see when they are no longer stolen.

Using xlsclients -la, I found a list of X apps, including those running in the background. I started killing them; some of the process terminations made my Gnome session break down, but I eventually found that shutting down the skypeforlinux process made CTRL+ALT+SHIFT+D work for me. 

It turns out that in Skype an extra "Debug" menu item appears if you press CTRL+ALT+SHIFT+D. It seems this behaviour cannot be disabled in Skype.

See dirkts hint which turned out to be very useful.

See dirkts hint which turned out to be very useful: Using "xlsclients -la", I found a list of X apps, including those running in the background. I started killing them; some of the process terminations made my Gnome session break down, but I eventually found that shutting down the skypeforlinux process made CTRL-ALT-SHIFT-d work for me. It turns out that in Skype, an extra "Debug" menu item appears, if you press CTRL-ALT-SHIFT-d. It seems this behaviour cannot be disabled in Skype.