Created attachment 184893 [details]
photo of the two-display layout

SUMMARY

I have my laptop set up to only use an external monitor (connected via HDMI), so the built-in screen is disabled and the external one is the primary and only display. I (almost) always use it this way, it's (almost) always plugged and I start up my computer this way.

Now I wanted to switch to two screens, so I opened Display Configuration, and I set the built-in screen to Enabled. It appeared to the left of the external one, as I wanted.
I didn't notice that the external one was still configured as the primary one - actually, I didn't notice because there was no indication of it whatsoever: the "Primary" radio button did not appear! I mistakenly, without thinking, assumed that the screen on the left was the primary one by definition.

I applied the configuration and what I got was the absolute nonsense that can be seen in the photos attached.

Both screens have the same resolutions of 1920 by 1080.

I don't even know how to describe in words the mess. Just look at the first photo. Notice the wallpaper image, especially on the seond (i.e. external) screen, and notice the task manager especially in the built-in (left) screen.
Also have a look at the second photo.

The fact that the task manager is in the right screen is I guess the expected consecuence of the fact that the right screen was the primary one, but still, there's a piece of task manager repeated on the left screen, and a piece of the left wallpaper repeated in the right screen.... Or it's just a piece of the right screen that is repeated on the right side of the left screen (which is confirmed by the second photo with an application open) - but the wallpaper looks correct on the left screen and not the right one....... again, a complete mess.

So I opened the Display Configuration settings again (that's when I noticed that the right screen was selected as the primary one) and I went back to the external-screen-only configuration. Then I switched back to the two-screen configuration, and then I was looking for the "Primary" selector and I thought I was stupid and couldn't find it, but I wasn't there. I had to (not only apply the configuration but also) close the settings and reopen them in order for the damn Primary selector to appear.

At this point I noticed that, this second time, the two screens were behaving fine. There was no repeated piece of screen on both displays or wallpaper weirdness, all was fine.

So it must be some random, "sometimes" glitch.

STEPS TO REPRODUCE
1.  On a laptop, configure the system to use only one external screen, with the builtin one turned off.
2. boot with that configuration
3. Open Display Configuration; in the dropdown, select the built-in screen and check the "Enabled" checkbox
=> Note that no "Primary" selector appears
4. Apply
=> Note that STILL no "Primary" selector appears
5. Look at the screens

OBSERVED RESULT
The mess I described

EXPECTED RESULT
The two screens should behave normally, the wallpaper should be repeated on both, the task manager should be only at the bottom of the screen selected as Primary (which will be  the external one as no opportunity was given so far to select anything else) and nothing should be duplicated on portions of both screens.



SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux 
KDE Plasma Version: 6.3.6
KDE Frameworks Version: 6.17.0
Qt Version: 6.9.1
Kernel Version: 6.6.101-1-MANJARO (64-bit)
Graphics Platform: Wayland
Processors: 12 × 12th Gen Intel® Core™ i7-1255U
Memory: 15.3 GiB of RAM
Graphics Processor: Intel® Iris® Xe Graphics
Manufacturer: ASUSTeK COMPUTER INC.
Product Name: Vivobook_ASUSLaptop X1502ZA_F1502ZA
System Version: 1.0