I don't like the code. Neither the one which used to be there, nor the new one. Centering on the screen is not the task of a window, but of the window manager and that code won't work on Wayland.

I'm not familiar with KDE, but it seems that's how krunner manages it's position too (method View::positionOnScreen in krunner/view.cpp), except that it doesn't use windowHandle() and derives current screen from cursor position, and takes struts in account

KWindowSystem doesn't seem to provide methods for manipulating window position, and neither I can find a method for getting screen geometry, so I have no idea how the code should look like

KRunner is different. It's not "a normal window". But the systemmonitor is a normal window.

It's just me the grumpy window manager speaking. We regularly get bug reports about window not opening on the screen the user expect. Reason for that is that apps do "smart things" like here. So if I complain in other areas I also have to complain here.

In D3484#90292, @aacid wrote:

@graesslin so how do we move forward with this, i.e. how can this work with Wayland?

It cannot. It is not the business of Windows to place themselves. That is also on X11 bad and breaking with e.g. filling window managers. It is also one of the main reasons why people complain that our window management sucks for multiple monitors. Windows placing themselves on multi screen mostly means positioning in a word way from user perspective.

So we should remove the setGeometry call altogether then?

Should we commit this then?

But it does no longer remember window size now (and for whatever reason, I cannot get the kwin rules to force an initial window size to work).

In D3484#238571, @cfeck wrote:

But it does no longer remember window size now (and for whatever reason, I cannot get the kwin rules to force an initial window size to work).

I thought the consensus was that the window shouldn't set it's properties (size and keepabove) and it's up to Kwin to set them
I've booted into neon dev unstable and kwin from master seems to struggle applying rules (I only took a quick peek, though, I'll need some time to test stable kwin with plasma-workspace from master), does Kwin apply rules to other windows fine for you?

I thought the consensus was that the window shouldn't set it's properties (size and keepabove) and it's up to Kwin to set them

It is the application that sets the window size. Since the initial size is likely wrong, applications usually remember sizes of windows and important dialogs. You removed that code that remembers the window size.

does Kwin apply rules to other windows fine for you?

For whatever reason, kwin refuses to save changes to the rules file. I had to kill kwin, edit .config/kwinrulesrc manually, and restart kwin to add a size rule for the systemmonitor. The rule now works.

In D3484#239893, @cfeck wrote:

It is the application that sets the window size. Since the initial size is likely wrong, applications usually remember sizes of windows and important dialogs.

If the application will set it's own geometry, then it'll still cause conflicts with (potential) KWin rules, e.g. Kwin applied it's size rule → systemmonitor resized itself → it looks like kwin failed to apply the rule. That's how I understood very first comments to the diff.
I can try & restore previous behavior just by saving window size as a size hint instead of letting the application resize itself, if that works then it should make both parties happy.

If the application will set it's own geometry, then it'll still cause conflicts with (potential) KWin rules

No, the KWin rules are a mean to override (or ignore) what applications set. An application shouldn't rely on KWin rules to deliver the user experience.

I've tried to restore old behavior, so:

• If I run KWindowConfig::restoreWindowSize from dialog's constructor, it doesn't work on multi-monitor setup very well. windowHandle()→screen() seems to be set to leftmost screen (which isn't even primary in my case; is this a bug?), so restoreWindowSize loads saved size for same display no matter on which display the window is opened. Which makes resizing the dialog on any other screen has no effect on the next time it's opened.
• If I run KWindowConfig::restoreWindowSize in a timer (like it used to be), it just messes up window positioning because the window is resized after it's been centered by KWin. Probably would mess up KWin rules too.

Patch to demonstrate the issue: