The only way I can find to mirror my screens on Wayland is to use the "Unify Outputs" button on the kscreen OSD. The "Replica of" combobox never appears in the KCM for me, even when I drag the screens on top of one another.
*** Bug 475284 has been marked as a duplicate of this bug. ***
Also the other way around: if a screen is already a replica of another screen, this cannot be changed with current UI.
*** Bug 477506 has been marked as a duplicate of this bug. ***
*** Bug 479930 has been marked as a duplicate of this bug. ***
The issue seems to be kcm.outputReplicationSupported being false in kscreen/kcm/ui/OutputPanel.qml
Will work on this (given the time frame, currently I plan to target 6.1 instead of 6.0).
A possibly relevant merge request was started @ https://invent.kde.org/plasma/libkscreen/-/merge_requests/180
Git commit 3fdbd0c8f8dc3627cf8de1bcab0c9c45d127a53e by Yifan Zhu. Committed on 22/01/2024 at 18:00. Pushed by fanzhuyifan into branch 'master'. backends/kwayland: enable output replication Output replication works under wayland. Mark it as supported so it shows up in kscreen kcm. FIXED-IN: 6.0 M +1 -1 backends/kwayland/waylandconfig.cpp https://invent.kde.org/plasma/libkscreen/-/commit/3fdbd0c8f8dc3627cf8de1bcab0c9c45d127a53e
Git commit 9faf6ee33aa8564ea9ab1224a0525f05e6ace39b by Yifan Zhu. Committed on 22/01/2024 at 18:03. Pushed by fanzhuyifan into branch 'Plasma/6.0'. backends/kwayland: enable output replication Output replication works under wayland. Mark it as supported so it shows up in kscreen kcm. FIXED-IN: 6.0 (cherry picked from commit 3fdbd0c8f8dc3627cf8de1bcab0c9c45d127a53e) M +1 -1 backends/kwayland/waylandconfig.cpp https://invent.kde.org/plasma/libkscreen/-/commit/9faf6ee33aa8564ea9ab1224a0525f05e6ace39b