The theme used by GTK3 CSD applications break the symmetry of the Breeze desktop. This can be fixed to an extent by adding rounded corners to the GTK3 themes with the same radius as that of the KWin window border. Also, adding a shadow effect to the Breeze-GTK3 theme will help in making these applications look more integrated to Plasma desktop than what they appear to be right now with no shadow and a sharp-corner for the border with a white outline. These changes might be subtle, but it will improve the user experience.
We can't do this because GTK rendering only works in compositors with Mutter-like rendering as of now. Until that is fixed, we can't do anything about visuals that rely on alpha transparency. We actually did a lot of careful tweaking in the last release to ensure that the theme would still look glitch-free in Plasma while retaining the rounded corners and resize borders in Mutter-based compositors, so the rounded corners are there, in fact, just not rendering in KWin because GTK won't allow it. If these long-standing bugs aren't fixed soon, the plan is to redo the theme to make headerbars appear like toolbars and force KWin's decorations on GTK CSD windows with a script. I know it's unpleasant at the moment, but there's really nothing we can do about it at this point.