SUMMARY
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NOTE: This may be a difficult nut to crack, but I make this report in the hope that if others have the same issue at least something may show up in the bug search. You are not alone brother/sister!

The mouse will move freely and show the correct cursor for the most part, but certain areas of the screen just will not accept mouse clicks or hover events at all, and in these cases the cursor will stay as an arrow. Examples:

1. Most common: Mouse stops working completely except movement. No buttons, scroll or hover events work. I have to resort to using keyboard to switch windows, click links in my browser or do anything at all.

2. Less common: Certain windows will work, while others will not. The windows that stop working will not accept mouse events at all, even for window titlebars or window edges so they cannot be moved or scaled.

3. Least common: Some areas of some windows, such as the bottom tool bar in Kate text editor will work while the rest of the window (main document area, title bar and window edges) will not work.

The session will start OK then after I have used it for some time the mouse will stop working. Sometimes it stops workine completely right away, while other times it will gradually degrade such that one window or part of one window  stops working first then progressively more parts stop working.

It has never happened to me that the mouse events start working again. The only way to rectify this error once it occurs is to restart the session.

The time it takes from the session starts until this mouse problem first occurs can be anything from 10 seconds to several weeks.

It seems that moving and/or scaling windows may accelerate the chance of the error occurring. I usually keep a fairly static window setup so this may explain why it sometimes takes a long time to show itself.

It should aslo be noted that I run Debian Guest inside Virtualbox on Windows 10 host. Virtualbox and Debian are both latest version as of this report.

Another issue I have seen is that plasmashell crashes alot. But this error occurs independently of wether plasma shell has crashed or not.

One thing that may be related is that I am running on a fairly wide monitor (Samsung S49A950UIU https://www.samsung.com/uk/monitors/high-resolution/s95ua-49-inch-dqhd-curved-ls49a950uiuxxu/#specs) It has 5,120 x 1,440 size. I have noticed that when I resize the virtualbox window to  a width above roughly 4k in width funny things start happening. This seem to be mildly related to the plasma shell crashes as well.

Since this happens very sporadically it is difficult to collect data (and I am trying to use the computer productively at work after all). So far this is the sparse data collected. I could probably exercise this more and try to reproduce the error in a debugger or similar. I am not familiar with KDE development but I have been avid Qt5/6 developer for many years if that is of any use.
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STEPS TO REPRODUCE
1. Start new session of KDE with debian inside vbox on win 11 on system with 5k monitor
2. Make sure guest editions are installed in vbox
3. Open a bunch of windows inside kde, such as filemanager, texteditor, browser etc
4. Resize vbox window to wider than 4k, and just move it around stressing the resizer
5. Resize and move windows around inside kde to stress resizing
6. If you are "lucky" the error will start to appear.

SOFTWARE/OS VERSIONS

Windows:
  Edition	Windows 11 Enterprise
  Version	21H2
  Installed on	‎3/‎1/‎2023
  OS build	22000.318
  Serial number	PF3YED6E
  Experience	Windows Feature Experience Pack 1000.22000.318.0


Linux/KDE Plasma:
  Linux debian 5.10.0-21-amd64 #1 SMP Debian 5.10.162-1 (2023-01-21) x86_64 GNU/Linux

  Operating System: Debian GNU/Linux 11
  KDE Plasma Version: 5.20.5
  KDE Frameworks Version: 5.78.0
  Qt Version: 5.15.2
  Kernel Version: 5.10.0-21-amd64
  OS Type: 64-bit
  Processors: 2 × 12th Gen Intel® Core™ i7-1260P
  Memory: 7,1 GiB of RAM
  Graphics Processor: SVGA3D; build: RELEASE; LLVM;