A variety of modes including python-mode.el will re-arrange the user's window layout when certain commands are issued.
For example, in python-mode.el, if the user has used Ctrl-x 3 to create a horizontal split with the code on the left, when the user issues Ctrl-c ! to create a REPL, python-mode.el will re-arrange the windows to have a vertical split.
Propose to nerf this behavior in the case that the user has manually re-arranged their windows, unless the mode sets a special flag indicating they are a special mode that only helps the user manage windows. This shouldn't break many modes, as the mode would need to work contingent on the user's window layout. Nearly all modes would continue to work based on the existence of their buffers.
Thus, if the user has entered Ctrl-x 1|2|3 at any time, modes would not be allowed to automatically re-arrange the windows.
This behavior works as expected for most use cases. For example, a user using Ctrl-x 2|3 to create a window before running M-x shell with the focus in that window, will only observe expected behavior.
There are special modes for this such as shackle, and several others. However, providing default behavior that makes sense to the user could clear up a lot of confusion.
In the case that a mode needs two windows, it is quite difficult for the user to manage this. For example, you must use use set-window-dedicated-p on all but two windows, so the new buffers will pop (basically randomly) in either of the two available slots (if two are available).
In the case that a mode needs two slots and intends to create them if not available, the special flag indicating that this is a mode that helps the user manage more complex window layouts could be set for that mode automatically on recognition of that behavior, as the mode can be considered as providing an IDE for the user.
As an initial first step for this feature, if the user has either a horizontal split or a vertical split, no mode without the special flag should be allowed to switch it the other way around.
Sincerely,
Brian