I've got to say that I partly agree with the above statement: as it stands, the window management in KDE, especially in regards to activities and plasmoids is broken, and has been so for many years.

I think, all this could work really great. It's got the right design and the right ambitions behind it, but it'll need fixing. Overall, I think KDE is amazing in terms of the customisability, a great range of native apps (my god, look at things like Kdenlive or Kile or KDE Connect), and an incredibly high level of usability. But when it comes to window management, it sucks.

I hope it'll be possible to find somebody at some point to look into this! Would love to do this myself but currently don't have the time.

I encountered this & was truly puzzled. I eventually, after little thoughts, figured out that the issue is only with windows without title-bar. I had removed some title-bars to save screen real-estate. I guess, the window class with proper key(window title) is required for this magic.