We recently added the gtk3-nocsd package to Kubuntu, in order to improve the visuals of GTK3 apps that use client-side decorations. This works, but this causes a minor UI issue that we can improve on.
By default, GNOME Application Style uses Breeze in System Settings > Application Style > GNOME Application Style (GTK) > Select a GTK3 theme. This style doesn't look as good for GTK3 apps anymore, now that we've turned the CSD headerbar into a toolbar.
Here's how it looks now, with gtk3-nocsd but the Breeze style:
And here's how it looks when we change Breeze to Default in System Settings > Application Style > GNOME Application Style (GTK) > Select a GTK3 theme:
Much better, no?