Most of the time, though, you run applications that aren't frozen. Sure, having resizing do something in that case is useful, but you don't want those black bar

Most of the time, though, you run applications that aren't frozen. Sure, having resizing do something in that case is useful, but you don't want those black bars to appear for the fraction of time between moving your mouse & the application redrawing, for every single frame of window resizing where the app didn't redraw in time (text reflowing, graphics initialization & other re-computation can often push resizing frame times above the 16ms of 60fps). (X11 and its server-side decorations appear to be able to handle this as-is - decoration redrawing is delayed until either the application redraws, or some timeout (iirc around a second or two?), at which point you get the ugly black bars)