Re: Gtk scrollbar: thumb too short

[Luc Teirlinck]

Owen Taylor wrote:

   While you may think of the scrollbar as a miniature view of the
   document, it is also presented to the user as a physical control,
   with obvious limits as to how far the scrollbar can be dragged.

I believe that this all boils down to the difference between (1) a
character and (2) a pixel based philosophy.  For efficiency reasons,
Emacs has no choice but to adopt (1).  Given that, it should do it
consistently, otherwise things would become unpredictable.

For (1) the thumb indicates the percentage of total "content"
contained in the part of the buffer that is represented above, on and
below the screen.  I believe that for (2) the thumb indicates how much
more upward and downward scrolling is possible, regardless of how much
extra content that is going to bring into view.

Start with an empty buffer in a window that has room for 60 lines of
regular character size and type:

a NEWLINE b

I believe that according to (1) the thumb should still cover the
entire length of the scrollbar, but according to (2), it should only
cover the top 60/61 of the scrollbar.  That is because there is now
one extra screen line we can scroll down to and according to the
visually based philosophy of (2), that matters.  According to the
character based philosophy of (1), there are no extra characters that
can be brought into view, even though extra scrolling is possible.

Now scroll the second line to the top.  One screen line, two
characters, scrolled of the screen.  We still have one third of the
characters on the screen, so for (1), the thumb should occupy the
bottom one third.  This is exactly what happens for the native
scrollbar.  (I do not see why the knob should "pop out of the trough".
The overscrolled piece contains no characters, so according to (1) its
size is 0, it does not really "exist", it is an optical illusion.)
For (2), I believe the thumb should occupy the bottom 60/61 of the
scrollbar.  Actually, I believe that for (2) mere scrolling should not
change the size of the thumb.

I believe that it is correct for the default behavior of GTK to be
pixel based, since it is more "intuitive" and covers the needs of more
applications.  I believe that the question is whether somehow one
could increase the "customizability" of the GTK scrollbar, to make it
possible, to have it, within Emacs, better approximate the native
scrollbar, without negatively affecting its interaction with
applications that want a pixel based behavior.  Alternatively, people
who prefer the consistently character based behavior (compared, not
with a consistently pixel based philosophy, but with some inconsistent
and unpredictable mixture) could use the native scrollbar (as I do).

Sincerely,

Luc.