When removed then added a new default panel, the whole system hang, it is not able to switch tty to kill plasmashell or restart sddm service. qt 5.6.x plasma-framework 5.21.0 plasma-workspace 5.6.2 plugin/plugout display outputs is able to reproduce the same issue, when mirror mode, plugout a output, the primary display's panel disappeared abnormally, wait for a minute, appeared again, then the whole desktop environment hang, not able to switch tty! Reproducible: Always
*** This bug has been marked as a duplicate of bug 356225 ***