Event: Make focus re-triggering not focus the original element back

If during a focus handler another focus event is triggered:

```js
elem1.on( "focus", function() {
	elem2.trigger( "focus" );
} );
```

due to their synchronous nature everywhere outside of IE the hack added in
gh-4279 to leverage native events causes the native `.focus()` method to be
called last for the initial element, making it steal the focus back. Since
the native method is already being called in `leverageNative`, we can skip that
final call.

This aligns with changes to the `_default` method for the `click` event that
were added when `leverageNative` was introduced there.

A side effect of this change is that now `focusin` will only propagate to the
document for the last focused element. This is a change in behavior but it also
aligns us better with how this works with native methods.

Fixes gh-4382
Closes gh-4813
Ref gh-4279

1 files changed, 6 insertions, 0 deletions

diff --git a/src/event.js b/src/event.js
index 4418fbbbf..48ee8d174 100644
--- a/src/event.js
+++ b/src/event.js
@@ -746,6 +746,12 @@ jQuery.each( { focus: "focusin", blur: "focusout" }, function( type, delegateTyp
 			return true;
 		},
 
+		// Suppress native focus or blur as it's already being fired
+		// in leverageNative.
+		_default: function() {
+			return true;
+		},
+
 		delegateType: delegateType
 	};
 } );