Event: Simulate focus/blur in IE via focusin/focusout

In IE (all versions), `focus` & `blur` handlers are fired asynchronously
but `focusin` & `focusout` are run synchronously. In other browsers, all
those handlers are fired synchronously. Asynchronous behavior of these
handlers in IE caused issues for IE (gh-4856, gh-4859).

We now simulate `focus` via `focusin` & `blur` via `focusout` in IE to avoid
these issues. This also let us simplify some tests.

This commit also simplifies `leverageNative` - with IE now using `focusin`
to simulate `focus` and `focusout` to simulate `blur`, we don't have to deal
with async events in `leverageNative`. This also fixes broken `focus` triggers
after first triggering it on a hidden element - previously, `leverageNative`
assumed that the native `focus` handler not firing after calling the native 
`focus` method meant it would be handled later, asynchronously, which
was not the case (gh-4950).

Fixes gh-4856
Fixes gh-4859
Fixes gh-4950
Closes gh-5223

Co-authored-by: Richard Gibson <richard.gibson@gmail.com>

src/event.js

@@ -1,9 +1,9 @@
 import jQuery from "./core.js";
-import document from "./var/document.js";
 import documentElement from "./var/documentElement.js";
 import rnothtmlwhite from "./var/rnothtmlwhite.js";
 import rcheckableType from "./var/rcheckableType.js";
 import slice from "./var/slice.js";
+import isIE from "./var/isIE.js";
 import acceptData from "./data/var/acceptData.js";
 import dataPriv from "./data/var/dataPriv.js";
 import nodeName from "./core/nodeName.js";
@@ -21,16 +21,6 @@ function returnFalse() {
 	return false;
 }
 
-// Support: IE <=9 - 11+
-// focus() and blur() are asynchronous, except when they are no-op.
-// So expect focus to be synchronous when the element is already active,
-// and blur to be synchronous when the element is not already active.
-// (focus and blur are always synchronous in other supported browsers,
-// this just defines when we can count on it).
-function expectSync( elem, type ) {
-	return ( elem === document.activeElement ) === ( type === "focus" );
-}
-
 function on( elem, types, selector, data, fn, one ) {
 	var origFn, type;
 
@@ -459,7 +449,7 @@ jQuery.event = {
 					el.click && nodeName( el, "input" ) ) {
 
 					// dataPriv.set( el, "click", ... )
-					leverageNative( el, "click", returnTrue );
+					leverageNative( el, "click", true );
 				}
 
 				// Return false to allow normal processing in the caller
@@ -511,10 +501,10 @@ jQuery.event = {
 // synthetic events by interrupting progress until reinvoked in response to
 // *native* events that it fires directly, ensuring that state changes have
 // already occurred before other listeners are invoked.
-function leverageNative( el, type, expectSync ) {
+function leverageNative( el, type, isSetup ) {
 
-	// Missing expectSync indicates a trigger call, which must force setup through jQuery.event.add
+	// Missing `isSetup` indicates a trigger call, which must force setup through jQuery.event.add
-	if ( !expectSync ) {
+	if ( !isSetup ) {
 		if ( dataPriv.get( el, type ) === undefined ) {
 			jQuery.event.add( el, type, returnTrue );
 		}
@@ -526,15 +516,13 @@ function leverageNative( el, type, expectSync ) {
 	jQuery.event.add( el, type, {
 		namespace: false,
 		handler: function( event ) {
-			var notAsync, result,
+			var result,
 				saved = dataPriv.get( this, type );
 
 			if ( ( event.isTrigger & 1 ) && this[ type ] ) {
 
 				// Interrupt processing of the outer synthetic .trigger()ed event
-				// Saved data should be false in such cases, but might be a leftover capture object
+				if ( !saved ) {
-				// from an async native handler (gh-4350)
-				if ( !saved.length ) {
 
 					// Store arguments for use when handling the inner native event
 					// There will always be at least one argument (an event object), so this array
@@ -543,28 +531,17 @@ function leverageNative( el, type, expectSync ) {
 					dataPriv.set( this, type, saved );
 
 					// Trigger the native event and capture its result
-					// Support: IE <=9 - 11+
-					// focus() and blur() are asynchronous
-					notAsync = expectSync( this, type );
 					this[ type ]();
 					result = dataPriv.get( this, type );
-					if ( saved !== result || notAsync ) {
 					dataPriv.set( this, type, false );
-					} else {
+
-						result = {};
-					}
 					if ( saved !== result ) {
 
 						// Cancel the outer synthetic event
 						event.stopImmediatePropagation();
 						event.preventDefault();
 
-						// Support: Chrome 86+
+						return result;
-						// In Chrome, if an element having a focusout handler is blurred by
-						// clicking outside of it, it invokes the handler synchronously. If
-						// that handler calls `.remove()` on the element, the data is cleared,
-						// leaving `result` undefined. We need to guard against this.
-						return result && result.value;
 					}
 
 				// If this is an inner synthetic event for an event with a bubbling surrogate
@@ -582,16 +559,11 @@ function leverageNative( el, type, expectSync ) {
 			} else if ( saved.length ) {
 
 				// ...and capture the result
-				dataPriv.set( this, type, {
+				dataPriv.set( this, type, jQuery.event.trigger(
-					value: jQuery.event.trigger(
+					saved[ 0 ],
-
-						// Support: IE <=9 - 11+
-						// Extend with the prototype to reset the above stopImmediatePropagation()
-						jQuery.extend( saved[ 0 ], jQuery.Event.prototype ),
 					saved.slice( 1 ),
 					this
-					)
+				) );
-				} );
 
 				// Abort handling of the native event
 				event.stopImmediatePropagation();
@@ -724,6 +696,29 @@ jQuery.each( {
 }, jQuery.event.addProp );
 
 jQuery.each( { focus: "focusin", blur: "focusout" }, function( type, delegateType ) {
+
+	// Support: IE 11+
+	// Attach a single focusin/focusout handler on the document while someone wants focus/blur.
+	// This is because the former are synchronous in IE while the latter are async. In other
+	// browsers, all those handlers are invoked synchronously.
+	function focusMappedHandler( nativeEvent ) {
+
+		// `eventHandle` would already wrap the event, but we need to change the `type` here.
+		var event = jQuery.event.fix( nativeEvent );
+		event.type = nativeEvent.type === "focusin" ? "focus" : "blur";
+		event.isSimulated = true;
+
+		// focus/blur don't bubble while focusin/focusout do; simulate the former by only
+		// invoking the handler at the lower level.
+		if ( event.target === event.currentTarget ) {
+
+			// The setup part calls `leverageNative`, which, in turn, calls
+			// `jQuery.event.add`, so event handle will already have been set
+			// by this point.
+			dataPriv.get( this, "handle" )( event );
+		}
+	}
+
 	jQuery.event.special[ type ] = {
 
 		// Utilize native event if possible so blur/focus sequence is correct
@@ -732,10 +727,15 @@ jQuery.each( { focus: "focusin", blur: "focusout" }, function( type, delegateTyp
 			// Claim the first handler
 			// dataPriv.set( this, "focus", ... )
 			// dataPriv.set( this, "blur", ... )
-			leverageNative( this, type, expectSync );
+			leverageNative( this, type, true );
+
+			if ( isIE ) {
+				this.addEventListener( delegateType, focusMappedHandler );
+			} else {
 
 				// Return false to allow normal processing in the caller
 				return false;
+			}
 		},
 		trigger: function() {
 
@@ -746,6 +746,16 @@ jQuery.each( { focus: "focusin", blur: "focusout" }, function( type, delegateTyp
 			return true;
 		},
 
+		teardown: function() {
+			if ( isIE ) {
+				this.removeEventListener( delegateType, focusMappedHandler );
+			} else {
+
+				// Return false to indicate standard teardown should be applied
+				return false;
+			}
+		},
+
 		// Suppress native focus or blur if we're currently inside
 		// a leveraged native-event stack
 		_default: function( event ) {

test/unit/event.js

@@ -2187,6 +2187,42 @@ QUnit.test( "focusin bubbles", function( assert ) {
 	jQuery( "body" ).off( "focusin.focusinBubblesTest" );
 } );
 
+QUnit.test( "focus does not bubble", function( assert ) {
+	assert.expect( 1 );
+
+	var done = assert.async(),
+		input = jQuery( "<input type='text' />" ).prependTo( "body" );
+
+	// focus the element so DOM focus won't fire
+	input[ 0 ].focus();
+
+	jQuery( "body" ).on( "focus.focusDoesNotBubbleTest", function() {
+		assert.ok( false, "focus doesn't fire on body" );
+	} );
+
+	input.on( "focus.focusDoesNotBubbleTest", function() {
+		assert.ok( true, "focus on the element" );
+	} );
+
+// Removed since DOM focus is unreliable on test swarm
+	// DOM focus method
+//	input[ 0 ].focus();
+
+	// To make the next focus test work, we need to take focus off the input.
+	// This will fire another focusin event, so set order to reflect that.
+//	jQuery( "#text1" )[ 0 ].focus();
+
+	// jQuery trigger, which calls DOM focus
+	input.trigger( "focus" );
+
+	input.remove();
+	jQuery( "body" ).off( "focus.focusDoesNotBubbleTest" );
+
+	setTimeout( function() {
+		done();
+	}, 50 );
+} );
+
 QUnit.test( "custom events with colons (trac-3533, trac-8272)", function( assert ) {
 	assert.expect( 1 );
 
@@ -2652,6 +2688,10 @@ QUnit.test( "element removed during focusout (gh-4417)", function( assert ) {
 	button[ 0 ].blur = function() {
 		jQuery.cleanData( [ this ] );
 		this.parentNode.removeChild( this );
+
+		// Redefine `blur` to avoid a hard crash in Karma tests that stop
+		// the test runner in case this test fails.
+		this.blur = jQuery.noop;
 	};
 
 	button[ 0 ].click();
@@ -3067,12 +3107,7 @@ QUnit.test( "focusout/focusin support", function( assert ) {
 	var focus,
 		parent = jQuery( "<div>" ),
 		input = jQuery( "<input>" ),
-		inputExternal = jQuery( "<input>" ),
+		inputExternal = jQuery( "<input>" );
-
-		// Support: IE <=9 - 11+
-		// focus and blur events are asynchronous; this is the resulting mess.
-		// The browser window must be topmost for this to work properly!!
-		done = assert.async();
 
 	parent.append( input );
 	jQuery( "#qunit-fixture" ).append( parent ).append( inputExternal );
@@ -3080,7 +3115,6 @@ QUnit.test( "focusout/focusin support", function( assert ) {
 	// initially, lose focus
 	inputExternal[ 0 ].focus();
 
-	setTimeout( function() {
 	parent
 		.on( "focus", function() {
 			assert.ok( false, "parent: focus not fired" );
@@ -3116,8 +3150,6 @@ QUnit.test( "focusout/focusin support", function( assert ) {
 	// then lose it
 	inputExternal[ 0 ].focus();
 
-		setTimeout( function() {
-
 	// DOM focus is unreliable in TestSwarm
 	if ( QUnit.isSwarm && !focus ) {
 		assert.ok( true, "GAP: Could not observe focus change" );
@@ -3131,10 +3163,6 @@ QUnit.test( "focusout/focusin support", function( assert ) {
 	// cleanup
 	parent.off();
 	input.off();
-
-			done();
-		}, 50 );
-	}, 50 );
 } );
 
 QUnit.test( "focus-blur order (trac-12868)", function( assert ) {
@@ -3142,17 +3170,10 @@ QUnit.test( "focus-blur order (trac-12868)", function( assert ) {
 
 	var order,
 		$text = jQuery( "#text1" ),
-		$radio = jQuery( "#radio1" ),
+		$radio = jQuery( "#radio1" );
-
-		// Support: IE <=9 - 11+
-		// focus and blur events are asynchronous; this is the resulting mess.
-		// The browser window must be topmost for this to work properly!!
-		done = assert.async();
 
 	$radio[ 0 ].focus();
 
-	setTimeout( function() {
-
 	$text
 		.on( "focus", function() {
 			assert.equal( order++, 1, "text focus" );
@@ -3172,7 +3193,6 @@ QUnit.test( "focus-blur order (trac-12868)", function( assert ) {
 	order = 0;
 	assert.equal( document.activeElement, $radio[ 0 ], "radio has focus" );
 	$text.trigger( "focus" );
-		setTimeout( function() {
 
 	// DOM focus is unreliable in TestSwarm
 	if ( QUnit.isSwarm && order === 0 ) {
@@ -3189,9 +3209,6 @@ QUnit.test( "focus-blur order (trac-12868)", function( assert ) {
 	// Clean up
 	$text.off();
 	$radio.off();
-			done();
-		}, 50 );
-	}, 50 );
 } );
 
 QUnit.test( "Event handling works with multiple async focus events (gh-4350)", function( assert ) {
@@ -3199,10 +3216,6 @@ QUnit.test( "Event handling works with multiple async focus events (gh-4350)", f
 
 	var remaining = 3,
 		input = jQuery( "#name" ),
-
-		// Support: IE <=9 - 11+
-		// focus and blur events are asynchronous; this is the resulting mess.
-		// The browser window must be topmost for this to work properly!!
 		done = assert.async();
 
 	input
@@ -3212,6 +3225,17 @@ QUnit.test( "Event handling works with multiple async focus events (gh-4350)", f
 			if ( remaining > 0 ) {
 				input.trigger( "blur" );
 			} else {
+
+				if ( QUnit.isIE ) {
+
+					// Support: <=IE 11+
+					// In IE, one of the blurs sometimes triggers a focus on body
+					// which in turn restores focus to the input, leading to 4 assertions
+					// firing instead of three. This only happens if other tests are
+					// running on the same test page. Avoid this issue in tests by removing
+					// the handler early.
+					input.off( "focus" );
+				}
 				done();
 			}
 		} )
@@ -3237,6 +3261,45 @@ QUnit.test( "Event handling works with multiple async focus events (gh-4350)", f
 	} );
 } );
 
+// Support: IE <=9 - 11+
+// focus and blur events are asynchronous.
+// The browser window must be topmost for this to work properly!!
+QUnit.test( "async focus queues properly (gh-4859)", function( assert ) {
+	assert.expect( 1 );
+
+	var $text = jQuery( "#text1" ),
+		$radio = jQuery( "#radio1" ),
+		done = assert.async();
+
+	$text.trigger( "focus" );
+	$radio.trigger( "focus" );
+	$text.trigger( "focus" );
+
+	setTimeout( function() {
+		assert.equal( document.activeElement, $text[ 0 ], "focus follows the last trigger" );
+		done();
+	}, 500 );
+} );
+
+// Support: IE <=9 - 11+
+// focus and blur events are asynchronous.
+// The browser window must be topmost for this to work properly!!
+QUnit.test( "async focus queues properly with blur (gh-4856)", function( assert ) {
+	assert.expect( 1 );
+
+	var $text = jQuery( "#text1" ),
+		done = assert.async();
+
+	$text.trigger( "focus" );
+	$text.trigger( "blur" );
+	$text.trigger( "focus" );
+
+	setTimeout( function() {
+		assert.equal( document.activeElement, $text[ 0 ], "focus-after-blur is respected" );
+		done();
+	}, 500 );
+} );
+
 QUnit.test( "native-backed events preserve trigger data (gh-1741, gh-4139)", function( assert ) {
 	assert.expect( 17 );
 
@@ -3246,12 +3309,7 @@ QUnit.test( "native-backed events preserve trigger data (gh-1741, gh-4139)", fun
 		targets = jQuery( parent[ 0 ].childNodes ),
 		checkbox = jQuery( targets[ 0 ] ),
 		data = [ "arg1", "arg2" ],
-		slice = data.slice,
+		slice = data.slice;
-
-		// Support: IE <=9 - 11+
-		// focus and blur events are asynchronous; this is the resulting mess.
-		// The browser window must be topmost for this to work properly!!
-		done = assert.async();
 
 	// click (gh-4139)
 	assert.strictEqual( targets[ 0 ].checked, false, "checkbox unchecked before click" );
@@ -3277,18 +3335,26 @@ QUnit.test( "native-backed events preserve trigger data (gh-1741, gh-4139)", fun
 		var type = event.type;
 		assert.deepEqual( slice.call( arguments, 1 ), data,
 			type + " handler received correct data" );
+
+		if ( QUnit.isIE && type === "focus" ) {
+
+			// Support: <=IE 11+
+			// In IE, one of the blurs sometimes triggers a focus on body
+			// which in turn restores focus to the input, leading to 4 assertions
+			// firing instead of three. This only happens if other tests are
+			// running on the same test page. Avoid this issue in tests by removing
+			// the handler early.
+			checkbox.off( "focus" );
+		}
 	} );
 	checkbox.trigger( "focus", data );
-	setTimeout( function() {
+
 	assert.strictEqual( document.activeElement, checkbox[ 0 ],
 		"element focused after focus event (default action)" );
 	checkbox.trigger( "blur", data );
-		setTimeout( function() {
+
 	assert.notEqual( document.activeElement, checkbox[ 0 ],
 		"element not focused after blur event (default action)" );
-			done();
-		}, 50 );
-	}, 50 );
 } );
 
 QUnit.test( "focus change during a focus handler (gh-4382)", function( assert ) {
@@ -3341,6 +3407,22 @@ QUnit.test( "trigger(focus) works after .on(focus).off(focus) (gh-4867)", functi
 	assert.equal( document.activeElement, input[ 0 ], "input has focus" );
 } );
 
+QUnit.test( "trigger(focus) works after focusing when hidden (gh-4950)", function( assert ) {
+	assert.expect( 1 );
+
+	var input = jQuery( "<input />" );
+
+	input.appendTo( "#qunit-fixture" );
+
+	input
+		.css( "display", "none" )
+		.trigger( "focus" )
+		.css( "display", "" )
+		.trigger( "focus" );
+
+	assert.equal( document.activeElement, input[ 0 ], "input has focus" );
+} );
+
 // TODO replace with an adaptation of
 // https://github.com/jquery/jquery/pull/1367/files#diff-a215316abbaabdf71857809e8673ea28R2464
 ( function() {