How to install/use sys-gui (collecting all related information):
“IMPORTANT: Do not let the screensaver (the lock screen) kick in before you can reset the password! The password will be unknown once the screensaver kicks in. You must reset the password with sudo passwd user each and every time you boot sys-gui (This is a bug that happens in sys-gui (but not sys-gui-gpu or sys-gui-vnc), that is documented in Make user password persistent in GUI domain · Issue #6740 · QubesOS/qubes-issues · GitHub )”