"scrollwheel" kind of widget

A type of widget I'm missing in GTK is something like the scrollbar, but
with no particular min/max values. This would be useful for scrolling a
canvas with a virtually infinite size, and for zooming (and possibly
other things). Irix (which uses something motif-like) has such a widget,
see the attached screenshot. (The small button below is used to reset to
the default value.)

Has this been discussed before? Are there any other (preferred) way to
solve the problem with existing widgets? Solutions I have seen (not
only in programs using GTK but any toolkit missing this widget) is non
optimal in my opinion, and include:

* The min/max values are set to the smallest/biggest possible. Thus this
  might be considered correct, it makes the scrollbar quite useless
  (since it scrolls the canvas WAY too much).

* The min/max values are constantly changed to be slightly
  smaller/bigger than the smallest/biggest value selected by the user.

* The min/max values are constantly changed to reflect a portion of a
  canvas that contains "something interesting".

* The scrollbar doesn't reflect the position and size of the canvas but
  always snaps back to middle position. (This is very similar to how the
  "scrollwheel" widget works, but I consider it a misuse of the

* The scrollbar indicates that the whole canvas is visible and only the
  stepper buttons can be used to scroll the canvas.

* There is no scrolling widget at all, and only the cursor keys and/or
  the mouse can be used to scroll the canvas.

Attachment: scrollwheel.png
Description: PNG image

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