How can Elisp code determine the previously selected window?

Question

I would like to implement an interactive function that, in the "canonical" case, selects the window that was selected before the currently selected one¹.

But my plan founders immediately on this question: how can my future function find out, to begin with, what the previously selected window is???

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^{1 There are a few edge cases, e.g. when no other window was previously selected, that I won't get into.}

Answer 1

Since Emacs 27.1, the previously selected window is recorded and can be obtained with old-selected-window. The previously selected window in each frame can be obtained with frame-old-selected-window. If you want older information, you can record it with a hook in window-selection-change-functions.

In older versions of Emacs, I don't think Emacs remembers any historical information about window selections by default, and there's no hook for window selection. You would have to advise at least select-window, and quite possibly other built-in functions that change the window selection.