I bit the bullet and spent some time yesterday installing XFCE and fiddling with it. And now it feels solid to me again too (it was never really shaky, just irritating in my use case). Focus reliably follows whatever window I alt-tab to, and I'm happy again.

I actually don't care what DE or window manager I use, my computer is just a place to hold a browser, a terminal window, and some development tools. All I require is that it stay out of my way and not make me notice it.