Remove implicit schedule of abstract methods in SpectatorScreen

This allows each implementation to have control over scheduling. Without
this, the solo implementation would not be able to handle quit events
while watching a player, as it would push a child (gameplay) screen to
the stack where the `SpectatorScreen` would usually be.

osu.Game/Screens/OnlinePlay/Multiplayer/Spectate/MultiSpectatorScreen.cs

@@ -228,7 +228,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
         {
         }
 
-        protected override void StartGameplay(int userId, SpectatorGameplayState spectatorGameplayState)
+        protected override void StartGameplay(int userId, SpectatorGameplayState spectatorGameplayState) => Schedule(() =>
         {
             var playerArea = instances.Single(i => i.UserId == userId);
 
@@ -242,9 +242,9 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
                 return;
 
             playerArea.LoadScore(spectatorGameplayState.Score);
-        }
+        });
 
-        protected override void QuitGameplay(int userId)
+        protected override void QuitGameplay(int userId) => Schedule(() =>
         {
             RemoveUser(userId);
 
@@ -252,7 +252,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Spectate
 
             instance.FadeColour(colours.Gray4, 400, Easing.OutQuint);
             syncManager.RemoveManagedClock(instance.SpectatorPlayerClock);
-        }
+        });
 
         public override bool OnBackButton()
         {

osu.Game/Screens/Play/SoloSpectatorScreen.cs

@@ -164,27 +164,33 @@ namespace osu.Game.Screens.Play
             automaticDownload.Current.BindValueChanged(_ => checkForAutomaticDownload());
         }
 
-        protected override void OnNewPlayingUserState(int userId, SpectatorState spectatorState)
+        protected override void OnNewPlayingUserState(int userId, SpectatorState spectatorState) => Schedule(() =>
         {
             clearDisplay();
             showBeatmapPanel(spectatorState);
-        }
+        });
 
-        protected override void StartGameplay(int userId, SpectatorGameplayState spectatorGameplayState)
+        protected override void StartGameplay(int userId, SpectatorGameplayState spectatorGameplayState) => Schedule(() =>
         {
             immediateSpectatorGameplayState = spectatorGameplayState;
             watchButton.Enabled.Value = true;
 
             scheduleStart(spectatorGameplayState);
-        }
+        });
 
         protected override void QuitGameplay(int userId)
         {
-            scheduledStart?.Cancel();
-            immediateSpectatorGameplayState = null;
-            watchButton.Enabled.Value = false;
+            // Importantly, don't schedule this call, as a child screen may be present (and will cause the schedule to not be run as expected).
+            this.MakeCurrent();
 
-            clearDisplay();
+            Schedule(() =>
+            {
+                scheduledStart?.Cancel();
+                immediateSpectatorGameplayState = null;
+                watchButton.Enabled.Value = false;
+
+                clearDisplay();
+            });
         }
 
         private void clearDisplay()

osu.Game/Screens/Spectate/SpectatorScreen.cs

@@ -129,7 +129,7 @@ namespace osu.Game.Screens.Spectate
             switch (newState.State)
             {
                 case SpectatedUserState.Playing:
-                    Schedule(() => OnNewPlayingUserState(userId, newState));
+                    OnNewPlayingUserState(userId, newState);
                     startGameplay(userId);
                     break;
 
@@ -173,7 +173,7 @@ namespace osu.Game.Screens.Spectate
             var gameplayState = new SpectatorGameplayState(score, resolvedRuleset, beatmaps.GetWorkingBeatmap(resolvedBeatmap));
 
             gameplayStates[userId] = gameplayState;
-            Schedule(() => StartGameplay(userId, gameplayState));
+            StartGameplay(userId, gameplayState);
         }
 
         /// <summary>
@@ -196,11 +196,12 @@ namespace osu.Game.Screens.Spectate
             markReceivedAllFrames(userId);
 
             gameplayStates.Remove(userId);
-            Schedule(() => QuitGameplay(userId));
+            QuitGameplay(userId);
         }
 
         /// <summary>
         /// Invoked when a spectated user's state has changed to a new state indicating the player is currently playing.
+        /// Thread safety is not guaranteed – should be scheduled as required.
         /// </summary>
         /// <param name="userId">The user whose state has changed.</param>
         /// <param name="spectatorState">The new state.</param>
@@ -208,6 +209,7 @@ namespace osu.Game.Screens.Spectate
 
         /// <summary>
         /// Starts gameplay for a user.
+        /// Thread safety is not guaranteed – should be scheduled as required.
         /// </summary>
         /// <param name="userId">The user to start gameplay for.</param>
         /// <param name="spectatorGameplayState">The gameplay state.</param>
@@ -215,6 +217,7 @@ namespace osu.Game.Screens.Spectate
 
         /// <summary>
         /// Quits gameplay for a user.
+        /// Thread safety is not guaranteed – should be scheduled as required.
         /// </summary>
         /// <param name="userId">The user to quit gameplay for.</param>
         protected abstract void QuitGameplay(int userId);