On a fresh install of Antergos 17.4 Plasma, if I install alternative window decorations (System Settings -> Application Style -> Window Decorations), such as Evolvere Light or two0ten (and almost any other), I can no longer re-size windows. The two-headed arrow that should appear near the corners of the window simply doesn’t appear.
The problem goes away when I switch back to Breeze, which is the default.
I tested this on Manjaro KDE Plasma and there I don’t get the same problem; I can resize the windows there. I don’t know if it’s significant, but activating, e.g. Evolvere in Antergos adds a rather fat border to the window, whereas I don’t see this in the Manjaro version.
All this seems to apply equally to GTK and Qt applications.
Additionally, changing windows decorations (System Settings -> Application Style -> Window Decorations) often freezes that window altogether and all I can do is click it closed and restart another instance of System Settings.
(I prefer Antergos, but I’d like to customize my Plasma…)