Propagate mouse motion events to boxes

internal/system/event.go

@@ -115,17 +115,18 @@ func (this *Hierarchy) HandleMouseMove (position image.Point) {
 	for _, dragSet := range this.drags {
 		for _, box := range dragSet {
 			if box.handleMouseMove() { break }
+			dragged = true
 		}
-		dragged = true
 	}
 	if dragged { return }
 
 	// TODO we can hover over multiple boxes at once. however, any way of
 	// detecting this involves several slice allocations every time we
 	// process a MouseMove event. perhaps we just ought to suck it up and do
-	// it.
+	// it. or perhaps doing *this* is the better way? we may never know.
 	box := this.boxUnder(position)
 	if box != nil {
+		box := this.considerMaskingParents(box)
 		this.hover(box)
 		box.handleMouseMove()
 	}