Tryton client 5.0 is acting really strange on KDE, probably due to gtk3 csd's

The 5.0 LTS version of the client is acting really strange on KDE. I think this probably due to the way it implements the GTK3 client-side window decorations.

I’ve attached below a few screenshots & explanations, which illustrate the issue. Is there a way to fix this with settings?

Here is the client window without any special Kwin-related settings. It looks normal enough, but if you look more closely, you can see that the text is a bit distorted & also if you look at the cursor position, the clickbox of the first toolbar item is offset down & to the right. This also applies to all of the UI elements, including items in the list.

Above is the client forced to run inside a Kwin frame. As you can see, there’s empty space around the GTK3 window frame inside. I think this is causing the issues.

If the client window is maximised full-screen this scaling/empty-space around frame issue goes away.

Edit: Oh also noticed that if I try to maximize the normal client window to half-full screen, there’s the same empty space around the side & bottom edges of the window & the same scaling issue. Clearly some bug in the way the client-side window decoration has been written. No similar issues on Windows (luckily).

Maybe it is linked to App Menu. As it is been retired from Gnome project, I created Issue 9115: App Menu retired by Gnome - Tryton issue tracker
If this does not help, I think it should be reported to GTK project because we do not do anything special in the client that could explain this behavior.

Oh. Sorry about that. I tested with a few other GTK3 & Python applications and this seems to be affect all of them. Will have to figure out, whether it’s a config, GTK3, or KDE issue.