track_canvas.c2w (x, y, wcx, wcy);
track_canvas.w2c (wcx, wcy, cx, cy);
nframes_t where = leftmost_frame + pixel_to_frame (x);
track_canvas.c2w (x, y, wcx, wcy);
track_canvas.w2c (wcx, wcy, cx, cy);
nframes_t where = leftmost_frame + pixel_to_frame (x);