>> (defun show-side-window ()
 >>    (display-buffer-in-side-window
 >>      (get-buffer "~/.emacs.d/init.el")
 >>      '((side . left)
 >>         (slot . 0)))
 >>    (display-buffer-in-side-window
 >>      (get-buffer "~/.emacs.d/init.el")
 >>      '((side . bottom)
 >>         (slot . 0))))
 >>
 >> (progn
 >>    (setq window-sides-vertical nil)
 >>    (show-side-window)) ;;; eval this first,  I expect left side window
 >> occupy full height, and yes.

No.  When 'window-sides-vertical' is nil, the bottom side window should
occupy full width.  The bug is here.

 >> (progn
 >>    (setq window-sides-vertical t)
 >>    (show-side-window)) ;; then eval this, I expect bottom window occupy
 >> full width, but not???
 >> ```
 >>
 >> I expect window-sides-vertical can control bottom side window occupy
 >> full frame width, but not. I think this is a bug.

OTOH this is OK.

The bug was introduced with the fix for Bug#73627, the release version
should not have it.  Please try the attached patch.

Thanks, martin