Event: Make trigger(focus/blur/click) work with native handlers

In `leverageNative`, instead of calling `event.stopImmediatePropagation()`
which would abort both native & jQuery handlers, set the wrapper's
`isImmediatePropagationStopped` property to a function returning `true`.
Since for each element + type pair jQuery attaches only one native handler,
there is also only one wrapper jQuery event so this achieves the goal:
on the target element jQuery handlers don't fire but native ones do.

Unfortunately, this workaround doesn't work for handlers on ancestors
- since the native event is re-wrapped by a jQuery one on each level of
the propagation, the only way to stop it for jQuery was to stop it for
everyone via native `stopPropagation()`. This is not a problem for
`focus`/`blur` which don't bubble, but it does also stop `click` on
checkboxes and radios. We accept this limitation.

Fixes gh-5015
Closes gh-5228

(cherry picked from commit 6ad3651)

src/event.js

@@ -559,8 +559,8 @@ function leverageNative( el, type, isSetup ) {
 					}
 
 				// 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.
+				// (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.
@@ -579,8 +579,16 @@ function leverageNative( el, type, isSetup ) {
 					this
 				) );
 
-				// Abort handling of the native event
-				event.stopImmediatePropagation();
+				// 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
+				// by stopping immediate propagation just on the jQuery event. However,
+				// the native event is re-wrapped by a jQuery one on each level of the
+				// propagation so the only way to stop it for jQuery is to stop it for
+				// everyone via native `stopPropagation()`. This is not a problem for
+				// focus/blur which don't bubble, but it does also stop click on checkboxes
+				// and radios. We accept this limitation.
+				event.stopPropagation();
+				event.isImmediatePropagationStopped = returnTrue;
 			}
 		}
 	} );

test/unit/event.js

@@ -3412,6 +3412,36 @@ QUnit.test( "trigger(focus) works after focusing when hidden (gh-4950)", functio
 	assert.equal( document.activeElement, input[ 0 ], "input has focus" );
 } );
 
+QUnit.test( "trigger(focus) fires native & jQuery handlers (gh-5015)", function( assert ) {
+	assert.expect( 3 );
+
+	var input = jQuery( "<input />" ),
+
+		// Support: IE 9 - 11+
+		// focus is async in IE; we now emulate it via sync focusin in jQuery
+		// but this test also attaches native handlers.
+		done = assert.async( 3 );
+
+	input.appendTo( "#qunit-fixture" );
+
+	input[ 0 ].addEventListener( "focus", function() {
+		assert.ok( true, "1st native handler fired" );
+		done();
+	} );
+
+	input.on( "focus", function() {
+		assert.ok( true, "jQuery handler fired" );
+		done();
+	} );
+
+	input[ 0 ].addEventListener( "focus", function() {
+		assert.ok( true, "2nd native handler fired" );
+		done();
+	} );
+
+	input.trigger( "focus" );
+} );
+
 // TODO replace with an adaptation of
 // https://github.com/jquery/jquery/pull/1367/files#diff-a215316abbaabdf71857809e8673ea28R2464
 ( function() {
@@ -3420,10 +3450,13 @@ QUnit.test( "trigger(focus) works after focusing when hidden (gh-4950)", functio
 			checkbox: "<input type='checkbox'>",
 			radio: "<input type='radio'>"
 		},
-		makeTestFor3751
+		function( type, html ) {
+			makeTestForGh3751( type, html );
+			makeTestForGh5015( type, html );
+		}
 	);
 
-	function makeTestFor3751( type, html ) {
+	function makeTestForGh3751( type, html ) {
 		var testName = "native-backed namespaced clicks are handled correctly (gh-3751) - " + type;
 		QUnit.test( testName, function( assert ) {
 			assert.expect( 2 );
@@ -3450,4 +3483,30 @@ QUnit.test( "trigger(focus) works after focusing when hidden (gh-4950)", functio
 			target.trigger( "click.fired" );
 		} );
 	}
+
+	function makeTestForGh5015( type, html ) {
+		var testName = "trigger(click) fires native & jQuery handlers (gh-5015) - " + type;
+		QUnit.test( testName, function( assert ) {
+			assert.expect( 3 );
+
+			var parent = supportjQuery( "<div class='parent'>" + html + "</div>" ),
+				input = jQuery( parent[ 0 ].firstChild );
+
+			parent.appendTo( "#qunit-fixture" );
+
+			input[ 0 ].addEventListener( "click", function() {
+				assert.ok( true, "1st native handler fired" );
+			} );
+
+			input.on( "click", function() {
+				assert.ok( true, "jQuery handler fired" );
+			} );
+
+			input[ 0 ].addEventListener( "click", function() {
+				assert.ok( true, "2nd native handler fired" );
+			} );
+
+			input.trigger( "click" );
+		} );
+	}
 } )();