bug#62707: 30.0.50; [PATCH] Non toolkit scroll bar handle

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Manuel Giraud <manuel@ledu-giraud.fr> writes:

> When I'm scrolling a file (with 'C-v'/'M-v' for example), the handle of
> the vertical scroll bar has its height that keeps changing even though
> the same amount of text is showed (there is no long line breaking
> involved).  I think it is distracting and weird but I may be missing
> something.

The size of the scroll bar represents the portion of text (in the entire
buffer) displayed in the current window.  So I'm afraid I cannot agree
to this change.

> While I was trying to understand how the scroll bar and its handle are
> drawn, I read this code where drawing a scroll bar is:
>
>        1- Fill the rectangle area above the handle (with background)
>        2- Fill the rectangle area of the handle (with foreground)
>        3- Fill the rectangle area below the handle (with background)

This is fast, and avoids much flicker.

> It seems more "natural" to me to:
>
>        1- Fill the entire scroll bar area (with background)
>        2- Fill the handle area on top of it (with foreground)

This will cause a lot of flicker on systems without XDBE, so I don't
agree to this either.

Last year, when I ported the scroll bar code to InputOnly windows, I
tried the change you suggested on an X terminal running NCDware 3.5, and
the scroll bar flickered heavily.