When I use my 4K monitor as a second monitor for my laptop (has a 1920x1080 screen), the 4K monitor will appear to be zoomed in 2x (right half of what should be visible is cut off) whether I'm using UI scaling or not.
The screen will also repeat the movements of previously rendered UIs that were shown on the 2nd monitor when any part of a UI updates, including non-Qt apps like Firefox. For instance, the broken images will continue to move whenever I hover over a UI element, whenever the text cursor blinks, when ever I type a letter or whenever I move a window.
STEPS TO REPRODUCE
1. Log into the "Plasma (Wayland)" or "Plasma (Full Wayland)" session
2. Move a window to the 2nd screen
Operating System: openSUSE Tumbleweed 20200621
KDE Plasma Version: 5.19.80
KDE Frameworks Version: 5.72.0
Qt Version: 5.15.0
Kernel Version: 5.7.2-1-default
OS Type: 64-bit
Processors: 4 × Intel® Core™ i7-6500U CPU @ 2.50GHz
Memory: 7.6 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 520
My monitor is an LG 27UK650-W
2x UI scaling just results in UIs that are 2x larger, but the screen still looks zoomed in 2x.
I'm using my Intel graphics. I have not tested Wayland with my Nvidia GTX 960m.
Output of `inxi -G` in case it's relevant:
Graphics: Device-1: Intel Skylake GT2 [HD Graphics 520] driver: i915 v: kernel
Device-2: NVIDIA GM107M [GeForce GTX 960M] driver: nvidia v: 440.82
Device-3: Realtek USB2.0 VGA UVC WebCam type: USB driver: uvcvideo
Display: x11 server: X.Org 1.20.8 driver: modesetting,nvidia resolution: 1: 1920x1080~60Hz 2: 3840x2160~30Hz
OpenGL: renderer: Mesa DRI Intel HD Graphics 520 (SKL GT2) v: 4.6 Mesa 20.1.1
a wild guess: does setting a wallpaper on the second monitor fix the problem?
(In reply to Vlad Zahorodnii from comment #2)
> a wild guess: does setting a wallpaper on the second monitor fix the problem?
Yes, that solved the flashing/repeating image problem. For some reason, the wallpaper is disabled every time I start a new session or restart plasmashell.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/318
Git commit 3e1443e003bf5fc0e20dd7de49505fd0227bdce4 by Vlad Zahorodnii.
Committed on 14/12/2020 at 14:09.
Pushed by vladz into branch 'master'.
wayland: Don't force a bpp of 24 for desktop windows
plasmashell provides the compositor buffers with an alpha channel and
without indicating the opaque region. Given that, kwin forces a bpp of
24 for desktop windows to improve performance. Unfortunately, there are
legit cases where plasma may attach a buffer that is not fully opaque,
for example when switching between activities.
Related: bug 421237
M +1 -1 waylandclient.cpp
Git commit 0d02ac8996db2d9fff8b3bf5e3e313a563a118af by David Edmundson.
Committed on 03/01/2021 at 22:18.
Pushed by davidedmundson into branch 'master'.
[shell] Set DesktopView to black when no contents are loaded
This is better than the ContainmentView default of clear as it means we
don't get garbage from the root window.
Related: bug 421237
M +2 -0 shell/desktopview.cpp