Re: The unwarranted scrolling assumption

[Eli Zaretskii]

> From: Lennart Borgman <address@hidden>
> Date: Fri, 18 Jun 2010 01:49:52 +0200
> Cc: address@hidden
> 
> > Eh, sorry, my confusion. It is not clear_glyph_matrix that failed, of
> > course. I just meant the reason we came to that line in try_scrolling.
> > What lead to the condition
> >
> >  w->cursor.vpos < 0
> >
> > BTW, looking at the code there are a couple of places where
> > w->cursor.vpos is set to -1. I guess the tests are just for if it is <
> > 0. Wouldn't it be better to set different negative values so it would
> > be easier to see where it failed? (That is normal error tracing.)
> 
> I notice now that this is clearly a separate issue, not related to
> speed. It is another bug.

As I tried to explain, this is not a bug, but expected behavior, at
least in general: when scrolling fails to bring point into the window,
the try_scrolling optimization fails, and other methods are tried
instead.

> Recentering happens whenever you are at the bottom of the screen if
> some conditions are met. I can reproduce it with this:
> 
> - Open window.c
> - Goto line 702:
> 
>                            + WINDOW_HEADER_LINE_HEIGHT (w)),
> 
> - Turn on wrap-to-fill-column-mode. Turn it off again.
> - Turn on visual-line-mode
> - I have (window-width) = 55, height 34.
> - Position to W in WINDOW (it is on a wrapped line)
> - Use C-L to get this to the bottom.
> - Press down arrow.
> 
> (Note: wrap-to-fill-column-mode justs adds add wrap-prefix here. Which
> is for some reason does not remove. Bug ;-( --
> wrap-to-fill-column-mode is of course part of nXhtml, but probably you
> can just add a fill prefix on the whole line that is equal to what the
> line indentation have.)
> 
> For me this recenters. And try_scrolling fails where I said before,
> around line 13581:
> 
>   /* Display the window.  Give up if new fonts are loaded, or if point
>      doesn't appear.  */
>   if (!try_window (window, startp, 0))
>     rc = SCROLLING_NEED_LARGER_MATRICES;
>   else if (w->cursor.vpos < 0)
>     {
>       clear_glyph_matrix (w->desired_matrix);
>       DebPrint (("try_scrolling SCROLLING_FAILED clear_glyph"));
>       rc = SCROLLING_FAILED;
>     }

Again, this is normal.  You cannot prevent recentering from kicking in
in some cases.