Always render when XR is enabled, even if no OS windows can draw

doc/classes/DisplayServer.xml

@@ -879,6 +879,12 @@
 				Registers callables to emit when the menu is respectively about to show or closed. Callback methods should have zero arguments.
 			</description>
 		</method>
+		<method name="has_additional_outputs" qualifiers="const">
+			<return type="bool" />
+			<description>
+				Returns [code]true[/code] if any additional outputs have been registered via [method register_additional_output].
+			</description>
+		</method>
 		<method name="has_feature" qualifiers="const">
 			<return type="bool" />
 			<param index="0" name="feature" type="int" enum="DisplayServer.Feature" />
@@ -1022,6 +1028,14 @@
 				Perform window manager processing, including input flushing. See also [method force_process_and_drop_events], [method Input.flush_buffered_events] and [member Input.use_accumulated_input].
 			</description>
 		</method>
+		<method name="register_additional_output">
+			<return type="void" />
+			<param index="0" name="object" type="Object" />
+			<description>
+				Registers an [Object] which represents an additional output that will be rendered too, beyond normal windows. The [Object] is only used as an identifier, which can be later passed to [method unregister_additional_output].
+				This can be used to prevent Godot from skipping rendering when no normal windows are visible.
+			</description>
+		</method>
 		<method name="screen_get_dpi" qualifiers="const">
 			<return type="int" />
 			<param index="0" name="screen" type="int" default="-1" />
@@ -1352,6 +1366,13 @@
 				[b]Note:[/b] [member ProjectSettings.audio/general/text_to_speech] should be [code]true[/code] to use text-to-speech.
 			</description>
 		</method>
+		<method name="unregister_additional_output">
+			<return type="void" />
+			<param index="0" name="object" type="Object" />
+			<description>
+				Unregisters an [Object] representing an additional output, that was registered via [method register_additional_output].
+			</description>
+		</method>
 		<method name="virtual_keyboard_get_height" qualifiers="const">
 			<return type="int" />
 			<description>

main/main.cpp

@@ -4108,7 +4108,7 @@ bool Main::iteration() {
 
 	RenderingServer::get_singleton()->sync(); //sync if still drawing from previous frames.
 
-	if (DisplayServer::get_singleton()->can_any_window_draw() &&
+	if ((DisplayServer::get_singleton()->can_any_window_draw() || DisplayServer::get_singleton()->has_additional_outputs()) &&
 			RenderingServer::get_singleton()->is_render_loop_enabled()) {
 		if ((!force_redraw_requested) && OS::get_singleton()->is_in_low_processor_usage_mode()) {
 			if (RenderingServer::get_singleton()->has_changed()) {

modules/openxr/openxr_interface.cpp

@@ -651,6 +651,10 @@ bool OpenXRInterface::initialize() {
 	// make this our primary interface
 	xr_server->set_primary_interface(this);
 
+	// Register an additional output with the display server, so rendering won't
+	// be skipped if no windows are visible.
+	DisplayServer::get_singleton()->register_additional_output(this);
+
 	initialized = true;
 
 	return initialized;
@@ -674,6 +678,8 @@ void OpenXRInterface::uninitialize() {
 		}
 	}
 
+	DisplayServer::get_singleton()->unregister_additional_output(this);
+
 	initialized = false;
 }
 

servers/display_server.cpp

@@ -759,6 +759,17 @@ DisplayServer::WindowID DisplayServer::get_focused_window() const {
 void DisplayServer::set_context(Context p_context) {
 }
 
+void DisplayServer::register_additional_output(Object *p_object) {
+	ObjectID id = p_object->get_instance_id();
+	if (!additional_outputs.has(id)) {
+		additional_outputs.push_back(id);
+	}
+}
+
+void DisplayServer::unregister_additional_output(Object *p_object) {
+	additional_outputs.erase(p_object->get_instance_id());
+}
+
 void DisplayServer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("has_feature", "feature"), &DisplayServer::has_feature);
 	ClassDB::bind_method(D_METHOD("get_name"), &DisplayServer::get_name);
@@ -997,6 +1008,10 @@ void DisplayServer::_bind_methods() {
 
 	ClassDB::bind_method(D_METHOD("is_window_transparency_available"), &DisplayServer::is_window_transparency_available);
 
+	ClassDB::bind_method(D_METHOD("register_additional_output", "object"), &DisplayServer::register_additional_output);
+	ClassDB::bind_method(D_METHOD("unregister_additional_output", "object"), &DisplayServer::unregister_additional_output);
+	ClassDB::bind_method(D_METHOD("has_additional_outputs"), &DisplayServer::has_additional_outputs);
+
 #ifndef DISABLE_DEPRECATED
 	BIND_ENUM_CONSTANT(FEATURE_GLOBAL_MENU);
 #endif

servers/display_server.h

@@ -53,6 +53,8 @@ class DisplayServer : public Object {
 	RID _get_rid_from_name(NativeMenu *p_nmenu, const String &p_menu_root) const;
 #endif
 
+	LocalVector<ObjectID> additional_outputs;
+
 public:
 	_FORCE_INLINE_ static DisplayServer *get_singleton() {
 		return singleton;
@@ -582,6 +584,10 @@ class DisplayServer : public Object {
 
 	virtual bool is_window_transparency_available() const { return false; }
 
+	void register_additional_output(Object *p_output);
+	void unregister_additional_output(Object *p_output);
+	bool has_additional_outputs() const { return additional_outputs.size() > 0; }
+
 	static void register_create_function(const char *p_name, CreateFunction p_function, GetRenderingDriversFunction p_get_drivers);
 	static int get_create_function_count();
 	static const char *get_create_function_name(int p_index);