Hi,

This is not directly related to bug#48916 but nonetheless derive 
from a same logic of window management.

Actually the user can choose if windmove will create a window when 
it encounter the edges of the frame. I propose to let the user 
eventually provides instead a function that will be triggered in 
that case.

The function will receive as arguments the `dir' and `window' 
bindings of `windmove-do-window-select' so it can adapt its 
behavior accordingly.

The simplest use case I found for this feature is to creates a 
dispatch function that choose from the direction to display eg, a 
dired side window on the left, a shell side window on the top, a 
message side window on the bottom, another thing on the right ; 
and that act as a toggle, eg :

windmove-left : creates a dired side window
windmove-left again : delete the dired side window

From here the function can be tuned again to behave differently in 
different contexts with eg, a local binding or by inspecting the 
WINDOW argument.