Event: Increase robustness of an inner native event in leverageNative

In Firefox, alert displayed just before blurring an element dispatches
the native blur event twice which tripped the jQuery logic if a jQuery blur
handler was not attached before the trigger call.

This was because the `leverageNative` logic part for triggering first checked if
setup was done before (which, for example, is done if a jQuery handler was
registered before for this element+event pair) and - if it was not - added
a dummy handler that just returned `true`. The `leverageNative` logic made that
`true` then saved into private data, replacing the previous `saved` array. Since
`true` passed the truthy check, the second native inner handler treated `true`
as an array, crashing on the `slice` call.

The same issue could happen if a handler returning `true` is attached before
triggering. A bare `length` check would not be enough as the user handler may
return an array-like as well. To remove this potential data shape clash, capture
the inner result in an object with a `value` property instead of saving it
directly.

Since it's impossible to call `alert()` in unit tests, simulate the issue by
replacing the `addEventListener` method on a test button with a version that
calls attached blur handlers twice.

Fixes gh-5459
Closes gh-5466
Ref gh-5236

src/event.js

@@ -519,14 +519,29 @@ function leverageNative( el, type, isSetup ) {
 			var result,
 				saved = dataPriv.get( this, type );
 
+			// This controller function is invoked under multiple circumstances,
+			// differentiated by the stored value in `saved`:
+			// 1. For an outer synthetic `.trigger()`ed event (detected by
+			//    `event.isTrigger & 1` and non-array `saved`), it records arguments
+			//    as an array and fires an [inner] native event to prompt state
+			//    changes that should be observed by registered listeners (such as
+			//    checkbox toggling and focus updating), then clears the stored value.
+			// 2. For an [inner] native event (detected by `saved` being
+			//    an array), it triggers an inner synthetic event, records the
+			//    result, and preempts propagation to further jQuery listeners.
+			// 3. For an inner synthetic event (detected by `event.isTrigger & 1` and
+			//    array `saved`), it prevents double-propagation of surrogate events
+			//    but otherwise allows everything to proceed (particularly including
+			//    further listeners).
+			// Possible `saved` data shapes: `[...], `{ value }`, `false`.
 			if ( ( event.isTrigger & 1 ) && this[ type ] ) {
 
 				// Interrupt processing of the outer synthetic .trigger()ed event
-				if ( !saved ) {
+				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
-					// will not be confused with a leftover capture object.
+					// There will always be at least one argument (an event object),
+					// so this array will not be confused with a leftover capture object.
 					saved = slice.call( arguments );
 					dataPriv.set( this, type, saved );
 
@@ -541,29 +556,35 @@ function leverageNative( el, type, isSetup ) {
 						event.stopImmediatePropagation();
 						event.preventDefault();
 
-						return result;
+						// Support: Chrome 86+
+						// 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
-				// (focus or blur), assume that the surrogate already propagated from triggering
-				// the native event and prevent that from happening again here.
-				// This technically gets the ordering wrong w.r.t. to `.trigger()` (in which the
-				// bubbling surrogate propagates *after* the non-bubbling base), but that seems
-				// less bad than duplication.
+				// If this is an inner synthetic event for an event with a bubbling
+				// surrogate (focus or blur), assume that the surrogate already
+				// propagated from triggering the native event and prevent that
+				// from happening again here.
 				} else if ( ( jQuery.event.special[ type ] || {} ).delegateType ) {
 					event.stopPropagation();
 				}
 
-			// If this is a native event triggered above, everything is now in order
-			// Fire an inner synthetic event with the original arguments
-			} else if ( saved ) {
+			// If this is a native event triggered above, everything is now in order.
+			// Fire an inner synthetic event with the original arguments.
+			} else if ( saved.length ) {
 
 				// ...and capture the result
-				dataPriv.set( this, type, jQuery.event.trigger(
-					saved[ 0 ],
-					saved.slice( 1 ),
-					this
-				) );
+				dataPriv.set( this, type, {
+					value: jQuery.event.trigger(
+						saved[ 0 ],
+						saved.slice( 1 ),
+						this
+					)
+				} );
 
 				// Abort handling of the native event by all jQuery handlers while allowing
 				// native handlers on the same element to run. On target, this is achieved

test/unit/event.js

@@ -3502,6 +3502,64 @@ QUnit.test( "trigger(focus) fires native & jQuery handlers (gh-5015)", function(
 	input.trigger( "focus" );
 } );
 
+QUnit.test( "duplicate native blur doesn't crash (gh-5459)", function( assert ) {
+	assert.expect( 4 );
+
+	function patchAddEventListener( elem ) {
+		var nativeAddEvent = elem[ 0 ].addEventListener;
+
+		// Support: Firefox 124+
+		// In Firefox, alert displayed just before blurring an element
+		// dispatches the native blur event twice which tripped the jQuery
+		// logic. We cannot call `alert()` in unit tests; simulate the
+		// behavior by overwriting the native `addEventListener` with
+		// a version that calls blur handlers twice.
+		//
+		// Such a simulation allows us to test whether `leverageNative`
+		// logic correctly differentiates between data saved by outer/inner
+		// handlers, so it's useful even without the Firefox bug.
+		elem[ 0 ].addEventListener = function( eventName, handler ) {
+			var finalHandler;
+			if ( eventName === "blur" ) {
+				finalHandler = function wrappedHandler() {
+					handler.apply( this, arguments );
+					return handler.apply( this, arguments );
+				};
+			} else {
+				finalHandler = handler;
+			}
+			return nativeAddEvent.call( this, eventName, finalHandler );
+		};
+	}
+
+	function runTest( handler, message ) {
+		var button = jQuery( "<button></button>" );
+
+		patchAddEventListener( button );
+		button.appendTo( "#qunit-fixture" );
+
+		if ( handler ) {
+			button.on( "blur", handler );
+		}
+		button.on( "focus", function() {
+			button.trigger( "blur" );
+			assert.ok( true, "Did not crash (" + message + ")" );
+		} );
+		button.trigger( "focus" );
+	}
+
+	runTest( undefined, "no prior handler" );
+	runTest( function() {
+		return true;
+	}, "prior handler returning true" );
+	runTest( function() {
+		return { length: 42 };
+	}, "prior handler returning an array-like" );
+	runTest( function() {
+		return { value: 42 };
+	}, "prior handler returning `{ value }`" );
+} );
+
 // TODO replace with an adaptation of
 // https://github.com/jquery/jquery/pull/1367/files#diff-a215316abbaabdf71857809e8673ea28R2464
 ( function() {