By primary display do you mean new app windows are created there? If so you can use window rules to force windows to be created on a particular screen.
You could put different windows on particular screens or just punt everything to the desired primary monitor.
This functionally is also available in i3wm/sway as for_window and as a stand alone application under x called devilspie2.
It's a pretty common feature. Windows, Gnome/X11 and KDE/X11 all have the concept of a "primary" monitor. I believe macOS does as well.
The idea is that things like the status bar, top bar, dock, etc, appear on the primary monitor unless configured otherwise.
Similarly, new windows preferentially open on the primary monitor.
This is existing functionality on all of these desktop environments.
In a multi-monitor setup, you designate a monitor as the primary monitor and the desktop automatically "does the right thing" even if you remove and reattach the primary display.
KDE on Wayland loses this functionality. The primary monitor is whichever monitor is detected first with no way to control it through configuration. The only workaround is to disable your secondary screen and then re-enable it, and you have to do that every time the desktop launches or the display configuration changes, which is a deal-breaker if you dock/undock regularly (or when the display server crashes).
Might there be hacky workarounds that partially fix some of these issues? Maybe. But it's a major functional regression and I'm not willing to invest that effort (especially, again, given the stability issues).
BTW, Gnome on Wayland preserves this functionality. But unfortunately, as I mentioned, Qt + Gnome leads to a whole host of other issues.
Logically you would just put the same dock on both monitors and tell it to move all new windows to DP-2 (or wherever), open an issue on their bug tracker asking for this feature to be reinstated and explaining why, and move on. This wouldn't so much be a hacky workaround as using a built in feature that achieves the same result.
In my opinion primary monitor is conflating 2 settings that perhaps ought to both exist in an easier to use that windows rules.
Open new windows on ____ selected from monitor with mouse cursor, same window as focused application, a selected monitor with the default being same monitor as focused application.
Always show panel in place of any other bar yes/no. Unchecked by default. You can have a panel on each monitor and put things of your choosing in each panel or even have a panel on top and below on the same monitor if you like. So what happens if you have a panel on your primary monitor 1 and also on your secondary monitors 2 and 3? Say you unplug the laptop and monitor 3 becomes the only monitor.
The most obvious and simple thing would be to show the same things on all 3 including showing a taskbar for the windows just on that particular monitor so that when you undock you can still do the same things. Windows does this for example the only difference is which bar has the tray.
The most theoretically correct would seem to be allowing you to set different bars for different configurations but this would be complicated and perhaps little used.
What I've suggested would be to give you the ability to set a primary bar that is always displayed hiding any secondary bars that normally appear on that display.
I think you're missing the entire point of my comment.
I'm not saying these things can't be resolved! They probably will be, eventually. I have every confidence that some day, KDE on Wayland will restore this now-missing feature.
I'm simply saying, through my personal laundry list of bugs and regressions, that Wayland is a long way off from ready for prime-time due to a wide range of issues that go well beyond the oft cited, very well-trod issue of the lack of screen sharing.
That's it.
And I genuinely look forward to the day those gaps are closed and I can use Wayland full time!
I just have a feeling that won't be until 2023 at the earliest.
I'd imagine this as far as things like docks are concerned? But the idea that I have to write specific windowing rules just to use my computer suggests this has a long way to go. It should 'just work' out of the box.
Just work doesn't mean it just works as you personally desire. It means just works in a consistent predictable fashion that is suitable for use not desirable for your taste or matching behavior on windows.
Personally I expect windows to show up on the same monitor as the currently focused window or the cursor and having it pop to a primary monitor would be from my perspective a defect. I would have to test but I think that outside of gnome this is actually how the majority of linux environments work out of the box.
You don't have set a windows rule in the settings gui just to use your computer. You have to set a windows rule in the settings gui for your computer to work in the fashion that you prefer. That you regard this setting as essential doesn't mean that it actually is just that you strongly prefer it.
You could put different windows on particular screens or just punt everything to the desired primary monitor.
This functionally is also available in i3wm/sway as for_window and as a stand alone application under x called devilspie2.