Attempt to improve safety of pushing matchmaking screens (ppy#37452)

Due to the push of the relevant screens being delayed it's possible that
the room goes away between the scheduling of the push and the actual
execution of the push.

This maybe closes ppy#37374 but my hopes
are not high.

Includes some extra cleanups I noticed along the way.

Author: Bartłomiej Dach

osu.Game.Tests/Visual/Matchmaking/TestSceneMatchmakingQueueScreen.cs

@@ -71,6 +71,22 @@ public void TestBasic()
             AddStep("change state to in room", () => queueScreen!.SetState(ScreenQueue.MatchmakingScreenState.InRoom));
         }
 
+        [Test]
+        public void TestDelayedRoomScreenPushDoesNotRunIfRoomIsLeftPrematurely()
+        {
+            AddStep("change state to in room then immediately leave room", () =>
+            {
+                queueScreen!.SetState(ScreenQueue.MatchmakingScreenState.InRoom);
+                MultiplayerClient.LeaveRoom();
+            });
+
+            // the queue screen waits 2 seconds between transitioning to `InRoom` state and actually pushing the relevant screen.
+            // if the room goes to `null` in that time, things die very hard.
+            // therefore the wait here is to check that things don't die very hard.
+            // if they do the test will throw an exception and fail.
+            AddWaitStep("wait a little bit", 10);
+        }
+
         private static double generateCount(double x, double mean, double stdDev, double amplitude)
         {
             return amplitude * Math.Exp(-Math.Pow(x - mean, 2) / (2 * Math.Pow(stdDev, 2))) + Random.Shared.Next(300);

osu.Game/Screens/OnlinePlay/Matchmaking/Queue/QueueController.cs

@@ -27,8 +27,6 @@ namespace osu.Game.Screens.OnlinePlay.Matchmaking.Queue
     ///
     /// Includes support for deferring to background.
     /// </summary>
-    /// <remarks>
-    /// This is initialised and cached in the <see cref="ScreenQueue"/> but can be used throughout the system via DI.</remarks>
     public partial class QueueController : Component
     {
         public readonly Bindable<ScreenQueue.MatchmakingScreenState> CurrentState = new Bindable<ScreenQueue.MatchmakingScreenState>();

osu.Game/Screens/OnlinePlay/Matchmaking/Queue/ScreenQueue.cs

@@ -90,6 +90,7 @@ public partial class ScreenQueue : OsuScreen
         private SampleChannel? waitingLoopChannel;
         private ScheduledDelegate? startLoopPlaybackDelegate;
         private DrawableSample waitingLoop = null!;
+        private ScheduledDelegate? pushScreenDelegate;
 
         private int? userRating;
 
@@ -390,14 +391,14 @@ private void onSelectedPoolChanged(ValueChangedEvent<MatchmakingPool?> e)
 
             if (e.NewValue == null)
             {
-                client.MatchmakingLeaveLobby();
+                client.MatchmakingLeaveLobby().FireAndForget();
                 return;
             }
 
             client.MatchmakingJoinLobbyWithParams(new MatchmakingJoinLobbyRequest
             {
                 PoolId = e.NewValue.Id
-            });
+            }).FireAndForget();
         }
 
         public override void OnEntering(ScreenTransitionEvent e)
@@ -465,6 +466,9 @@ public void SetState(MatchmakingScreenState newState)
             startLoopPlaybackDelegate?.Cancel();
             stopWaitingLoopPlayback();
 
+            pushScreenDelegate?.Cancel();
+            pushScreenDelegate = null;
+
             switch (newState)
             {
                 case MatchmakingScreenState.Idle:
@@ -599,7 +603,7 @@ public void SetState(MatchmakingScreenState newState)
 
                     using (BeginDelayedSequence(2000))
                     {
-                        Schedule(() =>
+                        pushScreenDelegate = Schedule(() =>
                         {
                             switch (poolType)
                             {