This logic is cleaner and produces better behavior when selecting by
dragging the mouse outside the bounds of the surface, previously when
doing this on the left side of the surface selections would include the
first cell of the next row, this is no longer the case.
This introduces methods on PageList.Pin which move a pin left or right
while wrapping to the prev/next row, or clamping to the ends of the row.
These need unit tests.
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