This problem affects KDE Plasma Wayland session on Arch Linux running
xwayland 1.19.6+13+gd0d1a694f-2 and it was already reported in KDE bug tracker
Kwin dev Martin Flöser said in comment 2 that it can be a bug in xwayland.
However Arch Linux running Gnome Wayland session and the same xwayland version is NOT affected.
Steps to reproduce:
start KDE Plasma Wayland session
open system settings, open "input devices", select "Mouse" section
select "left handed" radio button, click "Apply" button
open some xWayland window (Firefox, for example) to test the new mouse setting
Result: mouse is still working like in right handed mode.
Left button is still working as primary button and right button as auxiliary button.
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