Disallow clicking to toggle the checkbox of a theme override of type `Resource` to checked

These resources are external, but can be further categorized as follows:

1. Imported resource;
2. Embedded resource (no resource path, provided by the engine);
3. Other text-based resource.

Resources of different categorie may need different strategies.

Currently, for resource types, it is up to the user to decide what to do
next.  Clicking to toggle checkboxes is not allowed, but you can still
manipulate resources through the `EditorResourcePicker`'s context menu
(right-click on a resource in the inspector or click the down arrow icon
next to the resource to open the menu).

editor/inspector/editor_inspector.cpp

@@ -39,6 +39,7 @@
 #include "editor/editor_node.h"
 #include "editor/editor_string_names.h"
 #include "editor/editor_undo_redo_manager.h"
+#include "editor/gui/editor_toaster.h"
 #include "editor/gui/editor_validation_panel.h"
 #include "editor/inspector/add_metadata_dialog.h"
 #include "editor/inspector/editor_properties.h"
@@ -1062,6 +1063,16 @@ void EditorProperty::gui_input(const Ref<InputEvent> &p_event) {
 
 		if (check_rect.has_point(mpos)) {
 			accept_event();
+			if (!checked && Object::cast_to<Control>(object) && property_path.begins_with("theme_override_")) {
+				List<PropertyInfo> pinfo;
+				object->get_property_list(&pinfo);
+				for (const PropertyInfo &E : pinfo) {
+					if (E.type == Variant::OBJECT && E.name == property_path) {
+						EditorToaster::get_singleton()->popup_str(TTR("Toggling the checkbox is disabled for Resource properties. Modify the property using the resource picker instead."), EditorToaster::SEVERITY_WARNING);
+						return; // Disallow clicking to toggle the checkbox of type Resource to checked.
+					}
+				}
+			}
 			checked = !checked;
 			queue_redraw();
 			emit_signal(SNAME("property_checked"), property, checked);
@@ -1080,6 +1091,17 @@ void EditorProperty::gui_input(const Ref<InputEvent> &p_event) {
 void EditorProperty::_accessibility_action_click(const Variant &p_data) {
 	select();
 	if (checkable) {
+		if (!checked && Object::cast_to<Control>(object) && property_path.begins_with("theme_override_")) {
+			List<PropertyInfo> pinfo;
+			object->get_property_list(&pinfo);
+			for (const PropertyInfo &E : pinfo) {
+				if (E.type == Variant::OBJECT && E.name == property_path) {
+					EditorToaster::get_singleton()->popup_str(TTR("Toggling the checkbox is disabled for Resource properties. Modify the property using the resource picker instead."), EditorToaster::SEVERITY_WARNING);
+					return;
+				}
+			}
+		}
+
 		checked = !checked;
 		queue_redraw();
 		emit_signal(SNAME("property_checked"), property, checked);
@@ -5155,17 +5177,9 @@ void EditorInspector::_property_checked(const String &p_path, bool p_checked) {
 		} else {
 			Variant to_create;
 			Control *control = Object::cast_to<Control>(object);
-			bool skip = false;
 			if (control && p_path.begins_with("theme_override_")) {
 				to_create = control->get_used_theme_item(p_path);
-				Ref<Resource> resource = to_create;
-				if (resource.is_valid()) {
-					to_create = resource->duplicate();
-				}
-				skip = true;
-			}
-
-			if (!skip) {
+			} else {
 				List<PropertyInfo> pinfo;
 				object->get_property_list(&pinfo);
 				for (const PropertyInfo &E : pinfo) {