Cursor over the image: the wheel zooms about the pointer, and a drag pans,
both clamped to the contain-fit of the loaded bitmap so the photo can never
be lost off-screen. Cursor over an open parameter ruler: the wheel moves that
parameter by one step, clamped to its min/max. While the crop frame is being
dragged the wheel keeps resizing the crop instead, and APPLY puts the view
back on the fit.
3dtours
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