bug#9782: 24.0.90; move-to-window-line not taking header line into account

[Eli Zaretskii <eliz@gnu.org>]

> From: David Engster <deng@randomsample.de>
> Date: Tue, 18 Oct 2011 14:03:13 +0200
> 
> Recipe:
> 
> * emacs -Q
> 
> * Enter in scratch buffer:
> 
> (move-to-window-line (cdr (posn-actual-col-row (posn-at-point))))
> 
> and enter an additional newline so this is not the last line in the buffer.
> 
> * Move behind last bracket an hit C-x C-e
> 
> * Cursor will move to beginning of line, as expected.
> 
> * Now do M-: (setq header-line-format "") RET
> 
> * Evaluate the above again. You'll see that cursor now will move to the
>   beginning of the next line, which is wrong.
> 
> 
> This behavior occurs since rev. 106022, which fixed posn-actual-col-row
> when a header-line is active, but it seems move-to-window-line now has
> to be fixed as well.

Please provide some arguments as to why the current behavior is wrong.

posn-actual-col-row returns a _row_ derived from a pixel position,
while move-to-window-line accepts a _line_number_ starting from the
beginning of the text displayed in the window.  These two are not the
same.  Unless I'm mistaken, I see many users of move-to-window-line
that would break if we make the change you suggest.  E.g., what will
happen to code that does this:

  (move-to-window-line 0)

when there's a header line in the buffer, if your suggestion is
implemented?

Put it another way, the posn-* family of function deals with mouse
events, which are inherently oblivious to where text is displayed and
where we have window decorations.  By contrast, move-to-window-line
belongs to a different family of functions, one that deals with lines
of text.

Please show where this reasoning is wrong.