Re: Plasma 5.26 and multi-display support: Regression found

[Christian Volker <Volker.Christian@fh-hagenberg.at>]

Oh, i missed to say that I am still using X and not Wayland.


On 15.10.22 10:23, Christian Volker wrote:

First of all I would like to thank you for always fast delivering new KDE/Plasma versions to Debian (sid).

I've been using Debian-Sid for many (20)  years, so I know how to deal with broken packages and broken updates.
Therefore, this mail is not a complain but a report about my experiences - currently with the multi-display support in Plasma 5.26.

After upgrading to 5.26 a view days ago, I noticed that my external monitors no longer show any content - they only show a black screen.

About my system:

• Laptop: HP ZBook Furry G8, Nvidia T1200 (active), internal Intel-Graphics (deactivated)
  
• At home: HP Thunderbolt Dock G2 + an external Samsung monitor connected to the dock via DisplayPort.
• At work: HP Thunderbolt Dock G2 + two external Samsung monitors (different models than at home) each connected via DisplayPort to the dock.
• Mobile: ARZOPA Portable Monitor, 14 inch 1080 FHD connected directly to the laptop via USB-C.
• In lecture halls: Beamer connected to the laptop via HDMI.

Observed behavior:

• With Plasma 5.25.x and earlier: Everything works fine.
• Plasma (kscreen) saves the settings for the monitors and projectors (resolution, refresh rate, position and orientation) correctly and switches the monitors and beamer back on correctly when they are reconnected.
• With Plasma 5.26:
  
• Only the ARZOPA Portable Monitor correctly initialized again after reconnecting.
  
• All other displays (beamer via HDMI, Samsung monitors connected via display port to the Thunderbolt dock) remain dark.
• But I can initialize and bring them back to live correctly via the command line using xrandr - thus it is not Nvidia related.
  

In order to test whether it is due to Plasma 5.26 or not (maybe there is another error), yesterday I manually downgraded the entire Plasma 5.26 including the necessary KDE and Qt libraries back to 5.25.5. That took all afternoon, but after all the necessary, time consuming  preparations, the downgrade went smoothly.

After this downgrade, my system behaved correctly again as before. The displays connected via the Thunderbolt dock and DisplayPort were initialized correctly again and the projectors in the lecture halls (HDMI) also showed a picture again. So the problem must be related to the upgrade to Plasma 5.26. But I can't tell if the problem is related to Plasma 5.26 directly, or related to the KDE- or Qt-libs. To be honest, I do not have time to further investigate this regression in more detail.

Is this behavior also observed by other people?
Or is this behavior (already) known?
Or does this happen only on my system?

At least I'll stick with 5.25 for now...

All the best and thank you again for all the effort in always bringing the latest Plasma and KDE/Qt to Debian timely.
Volker