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Gnome and Windows provide CSD. Plasma choose another approach.

I think, It could be great to add terminal colors/images support to KWin, so apps could, for example colorize error messages placed in titlebar.

Another approach is to allow to place widget previews, so when user hover preview, tooltip bellow titlebar would be displayed (with dismiss and keep buttons). Application do not be informed about hover event, KWin will only show related widgets to preview and allow to interact with it.

These preview should be small.