bug#37840: Missing in the Emacs manuals:

[martin rudalics]

> Suppose I have buffer A open in frame A. Issuing occur->some word,
> the occur buffer pops up in its own frame, say frame B, as intended
> with the above customization. Moreover, frame B has input
> focus. Returning to frame A, without closing frame B, and issuing
> another time occur-> some word, frame B now shows the new occur
> buffer, as intended, but this time frame B lacks input focus. What
> goes wrong the second time?

Nothing, I suppose.  'occur' (I suppose that's the function you
invoke) calls 'occur-1' and that one simply does

            (display-buffer occur-buf)

which according to its doc-string does

  Display BUFFER-OR-NAME in some window, without selecting it.

IIUC the philosophy of 'occur' is given in its doc-string as

  The lines are shown in a buffer named `*Occur*'.
  It serves as a menu to find any of the occurrences in this buffer.

and programs usually don't preselect menus either.  The user has to
reach for the mouse or type some key to go there first ...

So the question is rather "what goes wrong the first time?".  The
answer to that is that, when 'display-buffer' creates a new frame, the
window manager usually also gives that new frame input focus.  Some
window managers allow to change that for all applications.  If you add
a non-nil 'no-focus-on-map' entry to your 'pop-up-frame-parameters',
Emacs will ask the window manager to not focus the new frame and
channces are that your window manager complies.

> Let me mention that if, in window.el, I add
>
> (x-focus-frame (window-frame window))
>
> at the very end of the defun "display-buffer-reuse-window", the
> problem goes away, i.e., in the above example, frame B gets input
> focus after every invocation of occur in frame A. How can I get this
> with a suitable customization on the "display-buffer-alist" level?

You can't.  I think the most simple way to achieve what you want is to
add a function to 'occur-hook' that does search for a window that
shows the current buffer and, if it finds such a window, invokes
'select-frame-set-input-focus' for that window's frame.

martin