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

(cherry picked from commit dbcffb396c)

src/event.js

@@ -778,6 +778,12 @@ 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
 	};
 } );

test/unit/event.js

@@ -3263,6 +3263,40 @@ QUnit.test( "native-backed events preserve trigger data (gh-1741, gh-4139)", fun
 	}, 50 );
 } );
 
+QUnit.test( "focus change during a focus handler (gh-4382)", function( assert ) {
+	assert.expect( 2 );
+
+	var done = assert.async(),
+		select = jQuery( "<select><option selected='selected'>A</option></select>" ),
+		button = jQuery( "<button>Focus target</button>" );
+
+	jQuery( "#qunit-fixture" )
+		.append( select )
+		.append( button );
+
+	select.on( "focus", function() {
+		button.trigger( "focus" );
+	} );
+
+	jQuery( document ).on( "focusin.focusTests", function( ev ) {
+		// Support: IE 11+
+		// In IE focus is async so focusin on document is fired multiple times,
+		// for each of the elements. In other browsers it's fired just once, for
+		// the last one.
+		if ( ev.target === button[ 0 ] ) {
+			assert.ok( true, "focusin propagated to document from the button" );
+		}
+	} );
+
+	select.trigger( "focus" );
+
+	setTimeout( function() {
+		assert.strictEqual( document.activeElement, button[ 0 ], "Focus redirect worked" );
+		jQuery( document ).off( ".focusTests" );
+		done();
+	} );
+} );
+
 // TODO replace with an adaptation of
 // https://github.com/jquery/jquery/pull/1367/files#diff-a215316abbaabdf71857809e8673ea28R2464
 ( function() {