Antergos system with Qt 5.6.0, Plasma 5.6.3 and KDE Frameworks 5.21.0 with the newest updates on today (on the moment of writing this message).
None of the tested by me applications are not restored after relogin/restart. I tested: Firefox, Thunderbird and new Opera. I think all of them are based on GTK2. All tested by me KF applications (konsole, kwrite, dolphin) are restored correctly (including virtual desktop #number).
I'm not sure if this is ksmserver problem or integration of Plasma in Antergos. I noticed that in Mageia Cauldron (not stable, but based on the newest Qt and Plasma) only thunderbird is restored, but no KF applications). Here (Antergos) only KF applications are restored.
I made also some test to be sure this is no GTK problem. I made some test relying on hardcoding in ksmserverrc file properly lines that forced running thunderbird. Result. After login thunderbird has been run. Unfortunately after relogin thunderbird was not run, because during closing the session my entry disappeared from ksmserverrc file.
Antergos system with Qt 5.6.0, Plasma 5.6.3 and KDE Frameworks 5.21.0 with the newest updates on today (on the moment of writing this message).
None of the tested by me applications are not restored after relogin/restart. I tested: Firefox, Thunderbird and new Opera. I think all of them are based on GTK2. All tested by me KF applications (konsole, kwrite, dolphin) are restored correctly (including virtual desktop #number).
I'm not sure if this is ksmserver problem or integration of Plasma in Antergos. I noticed that in Mageia Cauldron (not stable, but based on the newest Qt and Plasma) only thunderbird is restored, but no KF applications). Here (Antergos) only KF applications are restored.
I made also some test to be sure this is no GTK problem. I made some test relying on hardcoding in ksmserverrc file properly lines that forced running thunderbird. Result. After login thunderbird has been run. Unfortunately after relogin thunderbird was not run, because during closing the session my entry disappeared from ksmserverrc file.
For someone using openSUSE Leap it's working :/ Check this https://bugs.kde.org/show_bug.cgi?id=343518