[Drag-Drop] Prevent redundant drops

Mark default as prevented and check for prevention of
default when handling mctDrop events (which allow
views to follow up on drop gestures by positioning
objects within themselves) avoiding redundant drops
into embedded views. WTD-1233.

platform/features/layout/src/FixedController.js

@@ -249,6 +249,10 @@ define(
 
             // Position a panel after a drop event
             function handleDrop(e, id, position) {
+                // Don't handle this event if it has already been handled
+                if (e.isDefaultPrevented()) {
+                    return;
+                }
                 // Store the position of this element.
                 addElement({
                     type: "fixed.telemetry",
@@ -372,4 +376,4 @@ define(
 
         return FixedController;
     }
-);
+);