I'm writing shod2, a new version of my window manager.
I tried to play with auto-resizing of windows: https://0x0.st/-x9U.mp4
But it is too slow to redraw the window at every mouse motion...
So I'm back at resizing a frame of the window and then commiting the resizing once later: https://0x0.st/-x95.mp4

It's a dirty hack, but works.
That's how other window managers such as fvwm and *box do IIRC.