Attempt full editor reload on key count change

osu.Game.Rulesets.Mania/Edit/Setup/ManiaDifficultySection.cs

@@ -110,9 +110,31 @@ namespace osu.Game.Rulesets.Mania.Edit.Setup
             tickRateSlider.Current.BindValueChanged(_ => updateValues());
         }
 
+        private bool updatingKeyCount;
+
         private void updateKeyCount(ValueChangedEvent<float> keyCount)
         {
+            if (updatingKeyCount) return;
+
+            updatingKeyCount = true;
+
             updateValues();
+            editor.Reload().ContinueWith(t =>
+            {
+                if (!t.GetResultSafely())
+                {
+                    Schedule(() =>
+                    {
+                        changeHandler.RestoreState(-1);
+                        Beatmap.Difficulty.CircleSize = keyCountSlider.Current.Value = keyCount.OldValue;
+                        updatingKeyCount = false;
+                    });
+                }
+                else
+                {
+                    updatingKeyCount = false;
+                }
+            });
         }
 
         private void updateValues()

osu.Game/Localisation/EditorDialogsStrings.cs

@@ -49,6 +49,11 @@ namespace osu.Game.Localisation
         /// </summary>
         public static LocalisableString ContinueEditing => new TranslatableString(getKey(@"continue_editing"), @"Oops, continue editing");
 
+        /// <summary>
+        /// "The editor must be reloaded to apply this change. The beatmap will be saved."
+        /// </summary>
+        public static LocalisableString EditorReloadDialogHeader => new TranslatableString(getKey(@"editor_reload_dialog_header"), @"The editor must be reloaded to apply this change. The beatmap will be saved.");
+
         private static string getKey(string key) => $@"{prefix}:{key}";
     }
 }

osu.Game/Screens/Edit/Editor.cs

@@ -1231,6 +1231,27 @@ namespace osu.Game.Screens.Edit
             loader?.CancelPendingDifficultySwitch();
         }
 
+        public Task<bool> Reload()
+        {
+            var tcs = new TaskCompletionSource<bool>();
+
+            dialogOverlay.Push(new ReloadEditorDialog(
+                reload: () =>
+                {
+                    bool reloadedSuccessfully = attemptMutationOperation(() =>
+                    {
+                        if (!Save())
+                            return false;
+
+                        SwitchToDifficulty(editorBeatmap.BeatmapInfo);
+                        return true;
+                    });
+                    tcs.SetResult(reloadedSuccessfully);
+                },
+                cancel: () => tcs.SetResult(false)));
+            return tcs.Task;
+        }
+
         public void HandleTimestamp(string timestamp)
         {
             if (!EditorTimestampParser.TryParse(timestamp, out var timeSpan, out string selection))

osu.Game/Screens/Edit/ReloadEditorDialog.cs

@@ -0,0 +1,34 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Framework.Graphics.Sprites;
+using osu.Game.Overlays.Dialog;
+using osu.Game.Localisation;
+
+namespace osu.Game.Screens.Edit
+{
+    public partial class ReloadEditorDialog : PopupDialog
+    {
+        public ReloadEditorDialog(Action reload, Action cancel)
+        {
+            HeaderText = EditorDialogsStrings.EditorReloadDialogHeader;
+
+            Icon = FontAwesome.Solid.Sync;
+
+            Buttons = new PopupDialogButton[]
+            {
+                new PopupDialogOkButton
+                {
+                    Text = DialogStrings.Confirm,
+                    Action = reload
+                },
+                new PopupDialogCancelButton
+                {
+                    Text = DialogStrings.Cancel,
+                    Action = cancel
+                }
+            };
+        }
+    }
+}