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First visual change will be processed as normal and then blocked until the canvas renders the change. If further visual changes need processing then Editor::pre_render callback will schedule another expose/redraw/render. This prevents an issue where idle_visual_changer is called many times in response to events(keys/motion/etc) but the canvas does not get a chance to render any but the last one which results in a big pause/jump. This results in a more responsive canvas and in particular a smoother and more predictable zooming experience.
Please see the Ardour web site at http://ardour.org/ for all documentation..
For information on building ardour:
http://ardour.org/development.html