Layout breaks in Emacs 25.1

[kini]

Steps to reproduce:

• emacs -Q
• require shell-pop
• C-x 3 to split into left and right windows
• M-x shell-pop twice

Expected behavior:

After the first M-x shell-pop, a window appears at the bottom of the frame containing a shell. After the second M-x shell-pop, that window goes away and the layout of the frame returns to what it was previously (two windows side by side).

Actual behavior:

After the first M-x shell-pop, a window appears at the bottom of the frame, but the shell is loaded into the window on the right. After the second M-x shell-pop, the right side window is removed and what remains is a large top window and a small bottom window.

After the first M-x shell-pop:

After the second M-x shell-pop:

[MRFaria]

I think this happens because by default emacs25 changed the shell buffers to open in a new window instead of the same one.

Not sure how to fix shell-pop, but you can get the correct behavior with:
(push (cons "\\*shell\\*" display-buffer--same-window-action) display-buffer-alist)

Which makes the shell buffer open in the same window, which is what shell-pop expects

[rafipanoyan]

I have the same issue with Emacs 25.1.1

[articuluxe]

If anyone happens to use shackle, the following is an equivalent rule:
(add-to-list 'shackle-rules '("\\*shell\\*" :regexp t :same t))