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Re: Gtk scrollbar: thumb too short

From: Richard Stallman
Subject: Re: Gtk scrollbar: thumb too short
Date: Wed, 02 Apr 2003 04:19:18 -0500

    Your model is not simple either - you are just making it sound simple
    by neglecting the fact that there is some minimum size that you
    can make the scrollbar thumb.

The thumb size is quantized because the display is made of pixels.
Whatever model we use for the scroll bar will be distorted a little by
quantization.  This doesn't really affect any of the arguments.

    In fact, your doesn't work at all unless there is a difference between:

     - The minimum size the scrollbar thumb can under normal circumstances

This minimum is irrelevant except for very large files.  For instance,
etc/NEWS is almost half a meg, and yet its thumb is several pixels
high.  A file would have to be several meg before its thumb would have
the minimum possible height.

However, quantization does cause problems.  You cannot use mouse-2 to
scroll were you will.  In fact, a single pixel motion of the thumb in
etc/NEWS corresponds to more than one screenful.  If you scroll
through the file using mouse-2, there are parts you will not even see.
That is true regardless of how the scroll bar handles overscrolling.
However, it does have a relationship with the issue of overscrolling.

I compiled with some sort of athena widgets, and the behavior I get
for overscrolling is the one that you want.  If I slide the thumb all
the way to the bottom, my screen is blank.  But if I carefully
position the thumb one pixel up from the bottom, the last line is off
the bottom of the screen.  There is no place to put the thumb which
makes the end of the text visible.

With the behavior I am recommending, sliding the thumb to the bottom
would reliably make the last screenful of text visible.

    Many messages ago, you said:

     My suggestion is to display a thumb that rises from the bottom of the
     scroll bar, but is shorter than normal, as if the bottom of the thumb
     were hidden beyond the end of the scroll bar.  Others may think 
     of a better way to indicate this situation.

    I took that to understand that you wanted the bevelling drawn as
    if the scrollbar thumb was actually extending past the bottom
    of the scrollbar trough.

Actually I wasn't thinking about that question.  I don't know what GTK
scrollbars look like, so I did not know they have beveling.  Now that
you mention it, I think it would be desirable to omit the bottom
beveling, to indicate that the thumb virtually extends beyond the end
of the trough.

However, I am not saying that it should actually draw the thumb in the
space beyond the end of the trough.  I am saying that if it happens to
do that, it is ok.

    For GTK+, I really want to preserve the idea that the scrollbar
    consists of a thumb of some length that can be positioned between
    two extremes - at the top of the trough and at the bottom of the
    trough. For it to change length while dragging is a bit peculiar.
    For it to go past the ends of the trough, either literally or
    by shrinking as it is pushed against the end is completely outside
    that model.

When an application's internal concepts don't fit that model,
trying to force it to fit does not help.

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