Adding in cross-browser onbeforeunload support. Fixes #4106.

src/event.js

@@ -327,6 +327,8 @@ jQuery.event = {
 
 			}
 		}
+
+		return event.result;
 	},
 
 	props: "altKey attrChange attrName bubbles button cancelable charCode clientX clientY ctrlKey currentTarget data detail eventPhase fromElement handler keyCode layerX layerY metaKey newValue offsetX offsetY originalTarget pageX pageY prevValue relatedNode relatedTarget screenX screenY shiftKey srcElement target toElement view wheelDelta which".split(" "),
@@ -432,6 +434,21 @@ jQuery.event = {
 				}
 			},
 			special: {}
+		},
+		beforeunload: {
+			setup: function( data, namespaces, fn ) {
+				// We only want to do this special case on windows
+				if ( this.setInterval ) {
+					this.onbeforeunload = fn;
+				}
+
+				return false;
+			},
+			teardown: function( namespaces, fn ) {
+				if ( this.onbeforeunload === fn ) {
+					this.onbeforeunload = null;
+				}
+			}
 		}
 	}
 };