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

1 files changed, 58 insertions, 0 deletions

diff --git a/test/unit/event.js b/test/unit/event.js
index ac309d22d..111b1a18b 100644
--- a/test/unit/event.js
+++ b/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() {