PI 4 I've added display_hdmi_rotate=1 to /boot/config.txt and the display rotates. I've also tried =3 with the same results described below.
During boot the Raspberry Pi Desktop welcome splash is centered in the screen.
Once the desktop loads it's as if X is unaware of the change in aspect ratio, and the right half of the screen is off the edge of the monitor, while the bottom half of the monitor is blank.
I've tried changing the disable_overscan setting, I've also tried forcing the framebuffer height and width, but that merely increases the likelihood that X will simply crash. I've upped the memory split to 256.
It seems as if the underlying hardware is on board, but X isn't?