Refactor SystemTask state handling for resilience

State transitions now happen immediately where possible
This simplifies state management in general,
and prevents bugs such as the chime issue from occurring in the first place

src/displayapp/DisplayApp.cpp

@@ -255,9 +255,20 @@ void DisplayApp::Refresh() {
           isDimmed = true;
           brightnessController.Set(Controllers::BrightnessController::Levels::Low);
         }
-        if (IsPastSleepTime()) {
-          systemTask->PushMessage(System::Messages::GoToSleep);
-          state = States::Idle;
+        if (IsPastSleepTime() && uxQueueMessagesWaiting(msgQueue) == 0) {
+          PushMessageToSystemTask(System::Messages::GoToSleep);
+          // Can't set state to Idle here, something may send
+          // DisableSleeping before this GoToSleep arrives
+          // Instead we check we have no messages queued before sending GoToSleep
+          // This works as the SystemTask is higher priority than DisplayApp
+          // As soon as we send GoToSleep, SystemTask pre-empts DisplayApp
+          // Whenever DisplayApp is running again, it is guaranteed that
+          // SystemTask has handled the message
+          // If it responded, we will have a GoToSleep waiting in the queue
+          // By checking that there are no messages in the queue, we avoid
+          // resending GoToSleep when we already have a response
+          // SystemTask is resilient to duplicate messages, this is an
+          // optimisation to reduce pressure on the message queues
         }
       } else if (isDimmed) {
         isDimmed = false;
@@ -273,6 +284,9 @@ void DisplayApp::Refresh() {
   if (xQueueReceive(msgQueue, &msg, queueTimeout) == pdTRUE) {
     switch (msg) {
       case Messages::GoToSleep:
+        if (state != States::Running) {
+          break;
+        }
         while (brightnessController.Level() != Controllers::BrightnessController::Levels::Low) {
           brightnessController.Lower();
           vTaskDelay(100);
@@ -307,6 +321,9 @@ void DisplayApp::Refresh() {
         lv_disp_trig_activity(nullptr);
         break;
       case Messages::GoToRunning:
+        if (state == States::Running) {
+          break;
+        }
         if (settingsController.GetAlwaysOnDisplay()) {
           lcd.LowPowerOff();
         } else {