"JS error after using mousewheel without having moved the mouse before."

Update MouseWheel handler to use center of map if no mousemovement has 
been seen yet. (Closes #882)


git-svn-id: http://svn.openlayers.org/trunk/openlayers@4042 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Handler/MouseWheel.js

@@ -94,7 +94,17 @@ OpenLayers.Handler.MouseWheel = OpenLayers.Class(OpenLayers.Handler, {
                 // add the mouse position to the event because mozilla has a bug
                 // with clientX and clientY (see https://bugzilla.mozilla.org/show_bug.cgi?id=352179)
                 // getLonLatFromViewPortPx(e) returns wrong values
-                e.xy = this.mousePosition;
+                if (this.mousePosition) {
+                    e.xy = this.mousePosition;
+                } 
+                if (!e.xy) {
+                    // If the mouse hasn't moved over the map yet, then
+                    // we don't have a mouse position (in FF), so we just
+                    // act as if the mouse was at the center of the map.
+                    // Note that we can tell we are in the map -- and 
+                    // this.map is ensured to be true above.
+                    e.xy = this.map.getPixelFromLonLat(this.map.getCenter());
+                }
                 if (delta < 0) {
                    this.callback("down", [e, delta]);
                 } else {