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I known, window tabs was removed from KWin long ago.

It has many downsides, but maybe just made tabs behaves like window manager (from panel)?
This will correct behavior. Firstly, there is no need to set the same size or position or other criteria, like activity/virtual desktop/screen.

KWin will display down arrow after window title. When pressed, popup will be displayed with list of (each list as separate submenu):
1. List of windows of current application
2. List of windows from current desktop
3. List of windows from current activity
4. List of other's applications window list menu
5. Window actions + some other actions (like show desktop)
6. List of pined windows to current (by user)
7. Pin current window to other (contains list of each window)

Also, at top of 1-5 submenus, should stay overview button to show previews of each window in this submenu.
It will be similar to window group/tabs, but will be easier to implement, do not cause some problems and be better in many ways.

In future, each submenu could have implemented search field.