Dialog: Adjusted logic for finding the first tabbable element. Fixes …

…#5767 - On open, the first tabbable element inside the dialog was never being focused in favor of the dialog container.

ui/jquery.ui.dialog.js

@@ -324,12 +324,9 @@ $.widget("ui.dialog", {
 
 		// set focus to the first tabbable element in the content area or the first button
 		// if there are no tabbable elements, set focus on the dialog itself
-		$([])
-			.add(uiDialog.find('.ui-dialog-content :tabbable:first'))
-			.add(uiDialog.find('.ui-dialog-buttonpane :tabbable:first'))
-			.add(uiDialog)
-			.filter(':first')
-			.focus();
+		$(self.element.find(':tabbable').get().concat(
+			uiDialog.find('.ui-dialog-buttonpane :tabbable').get().concat(
+				uiDialog.get()))).eq(0).focus();
 
 		self._trigger('open');
 		self._isOpen = true;