If start with maximized frame, why do windows open top &bottom, not side by side?

Question

I have the following in my .emacs:

(add-to-list 'default-frame-alist '(fullscreen . maximized))

And

(add-hook 'window-setup-hook
      (lambda ()
        (find-file "FILE.org")
        (shell)))

However, these buffers open with one on top, and one on the bottom. As far as I can tell, the window-setup-hook is supposed to be run after all frame parameters, so these really should be side by side. If I manually maximize the frame, and then do these two commands in order, it opens them side by side. Any ideas?

EDIT: following Stefan's suggestion, I have submitted a bug report.

Answer 1

Emacs reads the .emacs after creating the initial frame, and after that it looks at default-frame-alist (and others) to see if it changed and then tries to update the initial frame accordingly. This update happens fairly late in the startup sequence, so apparently after running window-setup-hook.

If you're running Emacs≥27, you can set default-frame-alist in your ~/.emacs.d/early-init.el, so it takes effect even before the initial frame is created, thus completely avoiding these kinds of problems.