Bug 503962

SUMMARY
Windows from non-active activity are manifesting on my current activity. They
are invisible, but they are capturing mouse-clicks / focus. Often they
are on top of the windows on the current activity I'm trying to use and I need
to move them out of the way with alt-left click (my kwin move binding) or
resize them with alt-right click to get the the windows on the current
activity. They appear to follow normal window rules, as in, I can raise the
current activity windows on top of them, move them, resize them,


STEPS TO REPRODUCE
1. reboot / login, defaults to 1st activity
2. open a window (A) (any application)
3. switch activities
4. open another window (B)  on the 2nd activity (2)
5. switch back to the first activity (1) and attempt to interact with window A

OBSERVED RESULT
Window B from activity 2 will be invisible, and "on top of" window A. It will
prevent me from focusing window A until moved out of the way.


EXPECTED RESULT
I expect each window to remain on their own activities. This was prior behavior for years.


SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.3.4
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0
Kernel Version: 6.14.5-arch1-1 (64-bit)
Graphics Platform: X11
Processors: 16 × AMD Ryzen 7 2700X Eight-Core Processor
Memory: 31.3 GiB of RAM
Graphics Processor: NVIDIA GeForce RTX 3050


ADDITIONAL INFORMATION
I use focus follows mouse (sloppy).
Keyboard input is not captured initially, however if I move an invisible window it becomes focused and begins capturing keybaord input.
I can query the properties of the invisible window with xprops.
The invisible windows do not show up in the taskbar, or with alt-tab, or in the desktop overview.
I can move or resize them with alt-mouse, and when I switch back to the activity they are actually on, they appear moved and resized.
If I right-click on an invisible window, I see and can use the appropriate context menu from that application.
Each of my activities has 3 virtual desktops. The invisible windows don't come
from the corresponding desktop on the other activity, but instead whichever I
last had active. That is, if I'm on activity-1-desktop-3, and I switch to
activity-2, where desktop-2 is active, the invisible windows come from activity-1-desktop-3.

Assuming the bug is in kwin, I think I first observed it when updating from (6.2.3-1 -> 6.3.2.1-4)
Or possibly (6.3.2.1-4 -> 6.3.3.1-2)
I've just updated again (6.3.3.1-2 -> 6.3.4-4) and the problem persists.