This does not seem to be a problem for everyone nor for every XFCE theme, but sometimes it can be a real drag (no pun intended) to resize the windows in XFCE / XFWM4. For no apparent reason whatsoever, this seems to be known by the developers but marked as “wontfix” … #WTF
Anyway, hold down the (left) Alt key (PC keyboard), right click the mouse and then drag in the desired direction to “easily” resize the windows. This is a lot easier with an actual pointing device rather than a “touchpad”, but at least there’s a way.
You can also use Alt+Space and then press R to resize the current window.
This sort of “behavior” is one of (many small) things that annoys and frustrates people with “Linux Desktops”, and in all honesty, this should not be an issue in 2019! #FFS
ALT + [RIGHT-CLICK] + DRAG or ALT + [SPACE] followed by the [R] key