When an output is rotated with xrandr, the command-line utility prints
the pixel dimensions of the display as rotated, but the physical
dimensions of the display without rotation. As a result, when the
rotation is "left" or "right", they didn't match each other, and the
wrong scale was calculated. Check explicitly for this.
Each time the urxvt window is moved, check to see if the window is now
on a monitor with a different DPI. If so, calculate and set a new font
size so that the physical size of the text is unchanged. Works great
for laptops with HiDPI displays and external monitors; may require some
tweaking for other scenarios (e.g. laptop + presentation projector).
Scaling can be disabled by setting ENABLE_DPI_SCALING to 0 in the
extension source.
Note: This feature requires the xrandr command-line tool to be
installed.
Devin J. Pohly