[jackwilsdon]

Moving windows like this is already built into macOS but it's hidden behind a flag for some reason:

  defaults write -g NSWindowShouldDragOnGesture -bool true

You can then use Control+Command+Click to move windows from anywhere inside them. Sadly this doesn't provide resizing.

[atommclain]

FWIW I have a note in my dot files that for this to take effect you need to logout and then log back in, not sure if this is (still?) the case.

[oneeyedpigeon]

I think you still need to. I just tested and it doesn't take immediate effect. Which means I’ve now got to try to remember that keyboard shortcut and try it out the next time I restart...