Update "exit" flow when pressing back on Android

osu.Game/Screens/Menu/ExitConfirmOverlay.cs

@@ -3,6 +3,7 @@
 
 using osu.Framework.Input.Bindings;
 using osu.Framework.Input.Events;
+using osu.Game.Configuration;
 using osu.Game.Input.Bindings;
 using osu.Game.Overlays;
 
@@ -42,4 +43,26 @@ namespace osu.Game.Screens.Menu
             }
         }
     }
+
+    /// <summary>
+    /// An <see cref="ExitConfirmOverlay"/> that behaves as if the <see cref="OsuSetting.UIHoldActivationDelay"/> is always <c>0</c>.
+    /// </summary>
+    /// <remarks>This is useful for mobile devices using gesture navigation, where holding to confirm is not possible.</remarks>
+    public class NoHoldExitConfirmOverlay : ExitConfirmOverlay, IKeyBindingHandler<GlobalAction>
+    {
+        public new bool OnPressed(KeyBindingPressEvent<GlobalAction> e)
+        {
+            if (e.Repeat)
+                return false;
+
+            if (e.Action == GlobalAction.Back)
+            {
+                Progress.Value = 1;
+                Confirm();
+                return true;
+            }
+
+            return false;
+        }
+    }
 }

osu.Game/Screens/Menu/MainMenu.cs

@@ -3,6 +3,7 @@
 
 using System;
 using osu.Framework.Allocation;
+using osu.Framework.Audio;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
 using osu.Framework.Platform;
@@ -89,6 +90,14 @@ namespace osu.Game.Screens.Menu
                     }
                 });
             }
+            else if (host.CanSuspendToBackground)
+            {
+                AddInternal(exitConfirmOverlay = new NoHoldExitConfirmOverlay
+                {
+                    // treat as if the UIHoldActivationDelay is always 0. see NoHoldExitConfirmOverlay xmldoc for more info.
+                    Action = this.Exit
+                });
+            }
 
             AddRangeInternal(new[]
             {
@@ -148,6 +157,24 @@ namespace osu.Game.Screens.Menu
 
         private void confirmAndExit()
         {
+            if (host.CanSuspendToBackground)
+            {
+                // cancel the overlay as we're not actually exiting.
+                // this is the same action as 'onCancel' in `ConfirmExitDialog`.
+                exitConfirmOverlay.Abort();
+
+                // fade the track so the Bass.Pause() on suspend isn't as jarring.
+                const double fade_time = 500;
+                musicController.CurrentTrack
+                               .VolumeTo(0, fade_time, Easing.Out).Then()
+                               .VolumeTo(1, fade_time, Easing.In);
+
+                host.SuspendToBackground();
+
+                // on hosts that can only suspend, we don't ever want to exit the game.
+                return;
+            }
+
             if (exitConfirmed) return;
 
             exitConfirmed = true;