Well, just for fun I downloaded the current KDE Neon ISO and live booted a VM. Then, trying to reproduce the “bug” I actually had different things happening with each try and got a bit confused, but I think I figured out a few things.
First of, here’s a screencast of the actual “bug” happening:
You’ll notice there is a difference in behavior between clicking into the content window (which is the unexpected behavior) versus clicking the title bar (which is the expected one)
So, there’s our hint. If we go into system settings into “Window Behavior”, you’ll notice there’s a tab “Titlebar Actions” and another one “Window Actions”.
In “Window Actions” the Setting for “Left Click”, by default, is set to “Activate, pass click and raise on release”. At first, this sounds reasonable, but for some reason the “raise on release” part either does not seem to work, or I don’t understand what it’s supposed to mean.
After changing this setting to “Activate, raise and pass click” is when the behavior changes to how I (and OP) would actually expect it to.
So, by changing this setting this issue is “solved”, but I hope someone can explain this in a bit more detail.
And then, as an extra, I have another screencast where things just went completely wrong (still with the original setting):
As you can see, I activated the first Konsole window directly by opening the menu. This somehow lead to the file picker dialog to not block its parent and be able to be lowered behind it. I can’t really explain this one for now.