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The idea now is that a scroll group item can be added to the canvas which will causes its children to scroll in either or both directions (horizontal or vertical). There are few complications: the position() of the ScrollGroup is ambiguous depending on whether you want it with scroll taken into account or not, so Item::canvas_position() was added, which defaults to the same value as Item::position() but is overridden by ScrollGroup to return the position independent of scrolling. This method is used when translating between item/canvas/window coordinate systems. Note that the basic idea is that we MOVE the scroll group when a scroll happens. This mirrors what happens in the GnomeCanvas, where Nick Mainsbridge came up with a great idea that allowed unification of the time bar and track canvases.
Please see the Ardour web site at http://ardour.org/ for all documentation..
For information on building ardour:
http://ardour.org/development.html