Re: Gtk scrollbar: thumb too short

[Owen Taylor <otaylor@redhat.com>]

On Mon, 2003-04-07 at 12:57, Stefan Monnier wrote:
> > >     > 3.  Make the GTK scrollbar more customizable so that it can optionally
> > >     >     handle a character based approach and hence behave, within Emacs,
> > >     >     more like the native scrollbar than the pixel based variant of the
> > >     >     GTK scrollbar.
> > > 
> > >     I think this is the only reasonable solution.
> > > 
> > > I agree with Miles--this is the only right way.
> > > 
> > > Owen, would you please implement this facility in the GTK scroll bar?
> > 
> > I don't think there has been a satisfactory explanation of how you
> > are going to deal with minimum size issues; which for some themes
> > occur on quite moderately sized buffers. (As I said earlier, for
> > Red Hat's default theme, the minimum scrollbar thumb size is
> > ~30 pixels)
> 
> IIUC, the scrollbar widget has the following:
> 
> 	total-size
> 	minimum thumb size
> 	thumb size
> 	thumb position
> 
> Where minimum-thumb-size is (for all intents and purposes here) a constant.
> Let's simplify things and subtract minimum thumb size from the current
> thumb size and from the total, we get:
> 
> 	total-size
> 	thumb size
> 	thumb position
>
> where
> 	
> 	0 <= thumb-size <= total-size
> 	thumb-size + thumb-position <= total-size

This isn't a "simplification", it is a different algorithm
from what GTK+ uses. The scrollbar thumb size in GTK+
is:

 MAX (min_size, total_size * document_page_size/document_size)

Not:

 min_size + (total_size - min_size) * document_page_size / document_size

The difference between these two is substantial. It's
possible to fake your algorithm using GTK+'s range
code, but it requires knowing both the minimum size in
pixels (easy to determine as a style property) and the
trough size in pixels (a complicated function of style
properties and the allocated size of the range.)

Regards,
                                                Owen