(display-buffer-in-side-window

  (get-scratch-buffer-create)

  '((side . left)))

(split-window (frame-root-window) nil 'below)

(window-main-window)

Observe that (window-main-window) returns the selected window. However, the

window that was created by splitting below is not a side window, and it does not

descend from the selected window. This contradicts part of the docstring of

window-main-window:

"If FRAME has no side windows, return FRAME's root window. Otherwise, return

either an internal non-side window such that all other non-side windows on FRAME

descend from it, or the single live non-side window of FRAME."

Splitting a frame's root window in the presence of side windows seems to often

lead to such discrepancies. I have observed such behaviour in the packages hydra

(on ELPA, in its lv-window function), and shell-pop (from another package

archive).

Related to that: Is the following supposed to be an invariant for each frame,

and should it be considered a bug when packages violate it?

Invariant:

There exists a window MAIN such that for all live windows WIN (other than MAIN)

we have: WIN descends from MAIN if and only if it is not a side window.

This is something I assumed and was surprised to see violated.

Best,

Ferdinand