Comment 115 for bug 1181666

I think i found a way to reproduce this behavior:

Cause application to display "Application is not responding" dialog. For example rhythmbox causes this if you try to tag files two times, when the first tagging has not finished.
Another way is to run a X-forwarded application on a congested network connection.

When the dialog is open, you can switch between applications but mouse clicks inside windows are ignored.

Tested on Ubuntu 18.04 / Gnome 3.28