Allow the ui_menu action to trigger context menus in Tree

My right mouse button is dying. Not gonna replace it yet. :)

Added a test for this one feature, though no other Tree tests exist.

scene/gui/tree.cpp

@@ -3534,6 +3534,10 @@ void Tree::gui_input(const Ref<InputEvent> &p_event) {
 			}
 		}
 		accept_event();
+	} else if (p_event->is_action("ui_menu") && p_event->is_pressed()) {
+		// Show the context menu.
+		emit_signal(SNAME("item_mouse_selected"), get_local_mouse_position(), MouseButton::RIGHT);
+		accept_event();
 	}
 
 	if (allow_search && k.is_valid()) { // Incremental search

tests/scene/test_tree.h

@@ -0,0 +1,72 @@
+/**************************************************************************/
+/*  test_tree.h                                                           */
+/**************************************************************************/
+/*                         This file is part of:                          */
+/*                             GODOT ENGINE                               */
+/*                        https://godotengine.org                         */
+/**************************************************************************/
+/* Copyright (c) 2014-present Godot Engine contributors (see AUTHORS.md). */
+/* Copyright (c) 2007-2014 Juan Linietsky, Ariel Manzur.                  */
+/*                                                                        */
+/* Permission is hereby granted, free of charge, to any person obtaining  */
+/* a copy of this software and associated documentation files (the        */
+/* "Software"), to deal in the Software without restriction, including    */
+/* without limitation the rights to use, copy, modify, merge, publish,    */
+/* distribute, sublicense, and/or sell copies of the Software, and to     */
+/* permit persons to whom the Software is furnished to do so, subject to  */
+/* the following conditions:                                              */
+/*                                                                        */
+/* The above copyright notice and this permission notice shall be         */
+/* included in all copies or substantial portions of the Software.        */
+/*                                                                        */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,        */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF     */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. */
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY   */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,   */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE      */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                 */
+/**************************************************************************/
+
+#ifndef TEST_TREE_H
+#define TEST_TREE_H
+
+#include "scene/gui/tree.h"
+
+#include "tests/test_macros.h"
+
+namespace TestTree {
+
+TEST_CASE("[SceneTree][Tree] context menu detection") {
+    Tree *tree = memnew(Tree);
+    SceneTree::get_singleton()->get_root()->add_child(tree);
+    tree->grab_focus();
+
+    SUBCASE("[Tree] menu activation on empty") {
+        SIGNAL_WATCH(tree, "item_mouse_selected");
+
+        Ref<InputEventKey> event = memnew(InputEventKey);
+        event->set_keycode(Key::MENU);
+        event->set_pressed(true);
+        Input::get_singleton()->parse_input_event(event);
+
+        MessageQueue::get_singleton()->flush();
+
+        Vector2 mouse_pos = tree->get_local_mouse_position();
+        Array selection_args;
+        selection_args.push_back(mouse_pos);
+        selection_args.push_back(MouseButton::RIGHT);
+
+        Array test_args;
+        test_args.push_back(selection_args);
+
+        SIGNAL_CHECK("item_mouse_selected", test_args);
+
+        SIGNAL_UNWATCH(tree, "item_mouse_selected");
+        memdelete(tree);
+    }
+}
+
+} // namespace TestTree
+
+#endif // TEST_TREE_H

tests/test_main.cpp

@@ -115,6 +115,7 @@
 #include "tests/scene/test_path_2d.h"
 #include "tests/scene/test_primitives.h"
 #include "tests/scene/test_sprite_frames.h"
+#include "tests/scene/test_tree.h"
 #include "tests/scene/test_text_edit.h"
 #include "tests/scene/test_theme.h"
 #include "tests/scene/test_viewport.h"