Re: Let mode-line packages distinguish the selected-window

[martin rudalics <rudalics@gmx.at>]

 > And here is a weird edge case where doing so is not possible:
 >
 >    (add-hook 'pre-command-hook (lambda () (force-mode-line-update t)))
 >
 > Sure that is weird, but that's not the point.  With this hook function
 > we can observe the following behavior:
 >
 > 1. Enter minibuffer.
 > 2. Switch to another frame.  (Without leaving minibuffer first.)
 > 3. Observe that the window which was the selected window before
 >     (1) now uses mode-line-inactive but your code snippet returns t.

Running your example in an unmodified Emacs _without_ the hook you
sketched above gets me _two active_ mode-lines here (on a windowing
system that gives focus to a frame when the mouse enters it).  Running
it with the hook gets me an active mode-line for the window of the
focussed frame only.  I can't tell which of these is more distracting.

 > I think the only 100% sure way to be able to answer the first question
 > is to record that decision at the time when it is made.  Emphasize on
 > both "that decision" (not some closely related one) and "at that time",
 > i.e. when CURRENT_MODE_LINE_FACE_ID_3 is being called by
 > display_mode_lines, then that value has to be saved in a way that is
 > accessible from lisp.

Accessible only within the evaluation of 'mode-line-format'.

 > Now I am not saying this absolutely has to be done.  I am quite happy
 > with the above code.  But since I noticed this edge-case I though I
 > would point it out in case you decide that this is something we cannot
 > live with.

So maybe we should just provide a function called 'mode-line-active-p'
that returns non-nil when the window whose mode-line is just processed
is considered active (whether that decision is right or wrong) and not
talk about the selected window in the first place.

martin