bug#12419: Mouse click changes layout

[martin rudalics]

> Sorry, I don't understand what you mean by "drawing over a previous
> column/row".  Which "previous" column/row are we talking about?

For example, when we draw the mode-line of a window: Do we first clip
the glyphs on the bottom of the last proper line of the window or do we
draw them unclipped and afterwards draw the mode-line on top of it so it
obscures the lower part of the window line?

> A window must always start with a fully-visible line (unless it's the
> only line), so in that sense a window always starts at an integral
> number of lines.  But it doesn't have to _end_ with a fully-visible
> line.
>
> Does this explain why the above is not a problem?

Yes.  So the event reporting mechanism fully supports windows that
display partially visible lines.

>> Consider a two window frame, the upper window has 5 lines the lower
>> window has 6 lines but in fact both are shown with 5.5 lines.
>
> Can't happen: a window that displays 5.5 lines must have 6 lines, or
> else the glyphs for the last half-line will have no place in the glyph
> matrix.

Let's say the TTY equivalent of the upper window would display 5 lines.

>> Now I
>> enlarge the upper window by one line.  Currently this makes a 6 to 5
>> lines frame.  Would it make a 6.5 to 4.5 frame with the new code or a 6
>> to 5 lines frame?
>
> It's up to us.  The easiest (and also the least surprising, IMO) would
> be to resize from (5.5, 5.5) to (6.5, 4.5), i.e. by one full line.

In this case the TTY equivalent would display (6, 5) lines.

>> For implementing something like `count-screen-lines-to-pixels' and get
>> rid of that crazy loop where we calculate `pos-visible-in-window-p' and
>> resize the window.
>
> I think pos-visible-in-window-p is what you need.

Currently it loops calling `pos-visible-in-window-p' until the position
is visible.  How avoid that loop?

martin