Event: Use only one focusin/out handler per matching window & document

The `doc` variable in:
https://github.com/jquery/jquery/blob/3.4.1/src/event/focusin.js#L30
matched `document` for `document` & `window` for `window`, creating two
separate wrapper event handlers & calling handlers twice if at least one
`focusout` or `focusin` handler was attached on *both* `window` & `document`,
or on `window` & another regular node.

Also, fix the "focusin from an iframe" test to actually verify the behavior
from commit 1cecf64e5a - the commit that
introduced the regression - to make sure we don't regress on either front.

Fixes gh-4652
Closes gh-4656

src/event/focusin.js

@@ -27,7 +27,10 @@ if ( !support.focusin ) {
 
 		jQuery.event.special[ fix ] = {
 			setup: function() {
-				var doc = this.ownerDocument || this,
+
+				// Handle: regular nodes (via `this.ownerDocument`), window
+				// (via `this.document`) & document (via `this`).
+				var doc = this.ownerDocument || this.document || this,
 					attaches = dataPriv.access( doc, fix );
 
 				if ( !attaches ) {
@@ -36,7 +39,7 @@ if ( !support.focusin ) {
 				dataPriv.access( doc, fix, ( attaches || 0 ) + 1 );
 			},
 			teardown: function() {
-				var doc = this.ownerDocument || this,
+				var doc = this.ownerDocument || this.document || this,
 					attaches = dataPriv.access( doc, fix ) - 1;
 
 				if ( !attaches ) {

test/unit/event.js

@@ -2555,7 +2555,9 @@ testIframe(
 	function( assert, framejQuery, frameWin, frameDoc ) {
 		assert.expect( 1 );
 
-		var input = jQuery( frameDoc ).find( "#frame-input" );
+		var done = assert.async(),
+			focus = false,
+			input = jQuery( frameDoc ).find( "#frame-input" );
 
 		// Create a focusin handler on the parent; shouldn't affect the iframe's fate
 		jQuery( "body" ).on( "focusin.iframeTest", function() {
@@ -2563,23 +2565,56 @@ testIframe(
 		} );
 
 		input.on( "focusin", function() {
+			focus = true;
 			assert.ok( true, "fired a focusin event in the iframe" );
 		} );
 
 		// Avoid a native event; Chrome can't force focus to another frame
-		input.trigger( "focusin" );
+		input[ 0 ].focus();
-
-		// Must manually remove handler to avoid leaks in our data store
-		input.remove();
-
-		// Be sure it was removed; nothing should happen
-		input.trigger( "focusin" );
 
 		// Remove body handler manually since it's outside the fixture
 		jQuery( "body" ).off( "focusin.iframeTest" );
+
+		setTimeout( function() {
+
+			// DOM focus is unreliable in TestSwarm
+			if ( QUnit.isSwarm && !focus ) {
+				assert.ok( true, "GAP: Could not observe focus change" );
+			}
+
+			done();
+		}, 50 );
 	}
 );
 
+QUnit.test( "focusin on document & window", function( assert ) {
+	assert.expect( 1 );
+
+	var counter = 0,
+		input = jQuery( "<input />" );
+
+	input.appendTo( "#qunit-fixture" );
+
+	input[ 0 ].focus();
+
+	jQuery( window ).on( "focusout", function() {
+		counter++;
+	} );
+	jQuery( document ).on( "focusout", function() {
+		counter++;
+	} );
+
+	input[ 0 ].blur();
+
+	// DOM focus is unreliable in TestSwarm
+	if ( QUnit.isSwarm && counter === 0 ) {
+		assert.ok( true, "GAP: Could not observe focus change" );
+	}
+
+	assert.strictEqual( counter, 2,
+		"focusout handlers on document/window fired once only" );
+} );
+
 testIframe(
 	"jQuery.ready promise",
 	"event/promiseReady.html",