Here are a few different solutions to determine if a point is in a polygon with code examples. I think solution 3 might be applicable and faster.

Section: Determining if a point lies on the interior of a polygon http://paulbourke.net/geometry/polygonmesh/

Yup, that looks right, but it can be made even simpler in this case because one of the segments is always vertical.

  var prevOffset = prev.x-lowerRight.x;
  loc.x < lowerRight.x
  && (loc.y-prev.y)*-prevOffset-(loc.x-prev.x)*(upperRight.y-prev.y) <0
 && (loc.y-lowerRight.y)*prevOffset-(loc.x-lowerRight.x)*(prev.y-lowerRight.y) < 0

This is in fact equivalent to what's in jQuery.menu-aim.js, eg upperSlope < prevUpperSlope => upperSlope - prevUpperSlope < 0, then inline slope() and multiply through by the denominators.

I'd prefer the code above though, as there's no icky division by zero.

Got it, thanks! Should've thought about googling "hit test triangle".