io/router: deliver enter/leave events to transfer participants

When a drag and drop gesture is ongoing, let the potential target
handlers receive enter/leave events so that they can react to them (e.g.
highlight themselves when the dragged item is over them).

Fixes #321.

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

io/router/pointer.go

@@ -674,9 +674,20 @@ func (q *pointerQueue) deliverEnterLeaveEvents(p *pointerInfo, events *handlerEv
 	} else {
 		hits = q.opHit(e.Position)
 		if p.pressed {
-			// Filter out non-participating handlers.
+			// Filter out non-participating handlers,
+			// except potential transfer targets when a transfer has been initiated.
+			var transferSource *pointerHandler
+			if p.dataSource != nil {
+				transferSource = q.handlers[p.dataSource]
+			}
 			for i := len(hits) - 1; i >= 0; i-- {
-				if _, found := searchTag(p.handlers, hits[i]); !found {
+				tag := hits[i]
+				if transferSource != nil {
+					if _, ok := firstMimeMatch(transferSource, q.handlers[tag]); ok {
+						continue
+					}
+				}
+				if _, found := searchTag(p.handlers, tag); !found {
 					hits = append(hits[:i], hits[i+1:]...)
 				}
 			}