FIX: Set active input handler to InputManager when package is removed (ISXB-675)

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -24,6 +24,7 @@ however, it has to be formatted properly to pass verification tests.
 - Fixed pasting bindings into empty Input Action asset. [ISXB-1180](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1180)
 - Fixed missing '&' symbol in Control Scheme dropdown on Windows platform. [ISXB-1109](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1109)
 - Fixed icon scaling in Input Actions window.
+- Fixed an issue where removing the InputSystem package could lead to invalid input handling settings.
 - Fixed `ArgumentOutOfRangeException` when adding a new Control Scheme with any Device selected. [ISXB-1129](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1129)
 
 ### Changed

Packages/com.unity.inputsystem/InputSystem/Editor/InputSystemPackageControl.cs

@@ -0,0 +1,49 @@
+#if UNITY_EDITOR && UNITY_2020_2_OR_NEWER
+using System;
+using System.Collections.ObjectModel;
+using UnityEditor;
+using UnityEditor.PackageManager;
+
+namespace UnityEngine.InputSystem.Editor
+{
+    /// <summary>
+    /// Forces the Editor to restart if the InputSystem package is removed to activate and initialize it on the managed side.
+    /// Automatically sets "Project Settings > Player > Active Input Handling" to "Input Manager" once the package is removed.
+    /// </summary>
+    internal class InputSystemPackageControl
+    {
+        const string packageName = "com.unity.inputsystem";
+
+        [InitializeOnLoadMethod]
+        static void  SubscribePackageManagerEvent()
+        {
+            //There's a number of cases where it might not be called, for instance if the user changed the project manifest and deleted the Library folder before opening the project
+            UnityEditor.PackageManager.Events.registeringPackages += CheckForInputSystemPackageRemoved;
+        }
+
+        private static void CheckForInputSystemPackageRemoved(PackageRegistrationEventArgs packageArgs)
+        {
+            if (IsInputSystemRemoved(packageArgs.removed))
+                HandleInputSystemRemoved();
+        }
+
+        private static bool IsInputSystemRemoved(ReadOnlyCollection<UnityEditor.PackageManager.PackageInfo> packages)
+        {
+            foreach (var package in packages)
+            {
+                if (package.name == packageName)
+                    return true;
+            }
+            return false;
+        }
+
+        private static void HandleInputSystemRemoved()
+        {
+            //Set input handling to InputManager
+            EditorPlayerSettingHelpers.newSystemBackendsEnabled = false;
+            if (EditorUtility.DisplayDialog("The Unity Editor needs to be restarted", "You've removed the Input System package. This requires a restart of the Unity Editor.", "Restart the Editor", "Ignore (Not recommended)"))
+                EditorApplication.OpenProject(Environment.CurrentDirectory);
+        }
+    }
+}
+#endif