Eh? Before Yosemite was released in 2014, one window could span between a 1x and 2x monitor. I forget if it was always rendered at 1x, or if it was rendered at 2x and scaled for the 1x, but yeah it looked like crap on one or the other. Actually I think it switched depending on which monitor had more of the window?

But they only eliminated windows spanning across multiple monitors when they made spaces per-display.