Make enterleave plugin not depend on window level listener

Preventively fixing this independently. Tested on both that branch (works) and master (works, except for the case of moving into Comp from `html`, in which case this will have a duplicate `enter`, since one extra mouseout is captured from `html`).

Author: chenglou

src/browser/eventPlugins/EnterLeaveEventPlugin.js

@@ -56,7 +56,7 @@ var EnterLeaveEventPlugin = {
    * For almost every interaction we care about, there will be both a top-level
    * `mouseover` and `mouseout` event that occurs. Only use `mouseout` so that
    * we do not extract duplicate events. However, moving the mouse into the
-   * browser from outside will not fire a `mouseout` event. In this case, we use
+   * element from outside will not fire a `mouseout` event. In this case, we use
    * the `mouseover` top-level event.
    *
    * @param {string} topLevelType Record from `EventConstants`.
@@ -71,16 +71,22 @@ var EnterLeaveEventPlugin = {
       topLevelTarget,
       topLevelTargetID,
       nativeEvent) {
-    if (topLevelType === topLevelTypes.topMouseOver &&
-        (nativeEvent.relatedTarget || nativeEvent.fromElement)) {
-      return null;
-    }
     if (topLevelType !== topLevelTypes.topMouseOut &&
         topLevelType !== topLevelTypes.topMouseOver) {
-      // Must not be a mouse in or mouse out - ignoring.
+      // Unrelated event.
       return null;
     }
 
+    if (topLevelType === topLevelTypes.topMouseOver) {
+      // `fromElement`/`toElement` are IE's equivalent of `relatedTarget`.
+      var relatedTarget = nativeEvent.relatedTarget || nativeEvent.fromElement;
+      // See docblock above. Ignore all `mouseover` unless they come from
+      // outside of the top-level event listening container.
+      if (relatedTarget && ReactMount.getFirstReactDOM(relatedTarget) != null) {
+        return null;
+      }
+    }
+
     var win;
     if (topLevelTarget.window === topLevelTarget) {
       // `topLevelTarget` is probably a window object.