Brainy person needed for scrolling canvas problem!

[John Emmas <johne53 tiscali co uk>]

A program I'm working on uses a GTK main window to view an underlying gnome canvas object.  The canvas is 
wider than the part of it that's visible at any given time, so a horizontal scrollbar can be used to scroll 
the canvas, leftwards or rightwards.  The effect is similar to a multitrack audio tape.  Like the tape 
analogy, the user needs to be able to "fast-forward" and "rewind" (by dragging the scrollbar) but also, the 
canvas needs to scroll continuously in time (i.e. programatically) to simulate time-based movement (like a 
tape that's "playing").  There are two ways to achieve the "playing" effect:-

1) Every 1/30th second or thereabouts, erase all the objects on the canvas and redraw them, slightly to the 
left of their former positions.  This works quite well if the canvas only contains a small number of objects 
but if too many objects need to be redrawn, it soon becomes unwieldy and the movement ends up looking very 
jerky.

2) Draw all the objects onto a very wide canvas.  Every 1/30th second, reposition the viewport so that it's 
slightly to the right of its previous position (analogous to programatically setting the current position of 
the scrollbar).  This also works and gives a much smoother scrolling effect.

Now for the problem....  when in "playback" mode, I'd like the viewport to have a vertical "now line" 
(similar to the play head on a tape recorder).  At the moment, this is drawn onto the canvas.  Therefore 
every 1/30th second, it needs to be erased, then the viewport needs to be adjusted, then the "now line" needs 
to be redrawn.  This produces a very flickery now line.

In an ideal world, I'd like the now line to be a viewport object, rather than a canvas object.  The viewport 
would ideally contain some kind of transparent pane (with the now line drawn on it) and the canvas would be 
viewable through the pane's transparent part.  The pane would be treated like a foreground object (so as to 
be permanently visible) with a fixed position, relative to the viewport, whereas the canvas would have a 
variable position, relative to the viewport.

Off the top of my head, I can't think of any gtk widget that's designed for this purpose.  Has anyone ever 
seen a project with this kind of feature?  Or can anyone suggest a feasible strategy I could try?

Thanks,

John