Another KWin & GTk-CSD question - drop shadows and proper borders

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I spend a lot of time on a retro set-up with a self-built, hybrid Plasma4+5 DE on it. One of the few things it doesn’t do to my liking is the rendering of GTk3’s client-side window decoration. Those look like too much like an ancient Motif WM, with chunky borders around the windows and no drop-shadows.

I noticed that this is no longer the case with KF5 5.72 and Plasma 5.20, which isn’t too much newer than what I have.

Does anyone here know which component is responsible for this functionality? Is that KWin, KDecoration or the individual window themes?

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(I can’t seem to find relevant commit messages in the git histories of either of those projects.)

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To answer my own question: the main change was somewhere between 5.15.5 and 5.18.5 . Support for getting the window buttons to their custom configured locations must have been added later.