>> (display-buffer "*scratch*" '(display-buffer-in-side-window . ((side . right))))
 >> M-x split-root-window-right
 >>
 >> I would expect the new window to be to the left of the side window (I
 >> know this is counter-intuitive to splititng the root window, but it's
 >> also intuitive given the nature of side windows...)
 >>
 >> I don't know if this is intentional. I also don't know if it's a
 >> regression as it's not behavior I remember seeing until fairly recently.
 >
 > Martin, any comments?

There are two bugs.  The first one is that splitting a root window with
side windows is always an error unless a new side window on another side
should be made.  This should be fixed by the attached patch.

The second bug is that the 'split-root-window-...' functions probably
should split 'window-main-window' on that frame instead.  I'd rather
leave the fix for that to the people who wrote those functions.

martin