I fired up a live version of U-M 19.10 earlier today and was gratified to find that I could drag a window edge by clicking on the "shadow effect" that Marco provides by default and quite easily resize any window without having to edit a theme. So vkareh was correct in that this issue has received attention.
Edit: So from a userville perspective, the ability to easily drag a window edge has resulted in me adopting UM 19.10 beta as my daily driver. Sometimes it's the little things that count.