Scrolling in both directions at once is possible by meeting the required
distance thresholds to enable each direction separately.
+When a scroll gesture remains close to perfectly straight, it will be held to
+exact 90-degree angles; but if the gesture moves diagonally, it is free to
+scroll in any direction.
+
Two-finger scrolling requires the touchpad to track both touch points with
reasonable precision. Unfortunately, some so-called "semi-mt" touchpads can
only track the bounding box of the two fingers rather than the actual