FIX: Make displayIndex usable for PenState

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -55,6 +55,7 @@ however, it has to be formatted properly to pass verification tests.
 - Fixed potential crash on Mac when using stale references to deleted InputDevice objects [ISXB-606](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-606).
 - Fixed conditional compilation for non-editor analytics on platforms not enabling analytics.
 - Fixed simulated touch input not working with PlayerInput component [ISXB-483](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-483).
+- Fixed unused PenState information to determine the displayIndex on platforms providing it. (PLAT-10123)
 
 ### Changed
 - Renamed editor Resources directories to PackageResources to fix package validation warnings.

Packages/com.unity.inputsystem/InputSystem/Devices/Pen.cs

@@ -98,8 +98,10 @@ public struct PenState : IInputStateTypeInfo
         [FieldOffset(32)]
         public ushort buttons;
 
-        // Not currently used, but still needed in this struct for padding,
-        // as il2cpp does not implement FieldOffset.
+        /// <summary>
+        /// The index of the display that was touched.
+        /// </summary>
+        [InputControl(name = "displayIndex", displayName = "Display Index", layout = "Integer")]
         [FieldOffset(34)]
         ushort displayIndex;
 
@@ -379,6 +381,7 @@ protected override void FinishSetup()
             inRange = GetChildControl<ButtonControl>("inRange");
             tilt = GetChildControl<Vector2Control>("tilt");
             twist = GetChildControl<AxisControl>("twist");
+            displayIndex = GetChildControl<IntegerControl>("displayIndex");
             base.FinishSetup();
         }
     }

Packages/com.unity.inputsystem/InputSystem/Plugins/InputForUI/InputSystemProvider.cs

@@ -420,7 +420,7 @@ void OnPointerPerformed(InputAction.CallbackContext ctx)
                 m_SeenPenEvents = true;
 
             var positionISX = ctx.ReadValue<Vector2>();
-            var targetDisplay = asPointerDevice != null ? asPointerDevice.displayIndex.ReadValue() : (asTouchscreenDevice != null ? asTouchscreenDevice.displayIndex.ReadValue() : 0);
+            var targetDisplay = asPointerDevice != null ? asPointerDevice.displayIndex.ReadValue() : (asTouchscreenDevice != null ? asTouchscreenDevice.displayIndex.ReadValue() : (asPenDevice != null ? asPenDevice.displayIndex.ReadValue() : 0));
             var position = ScreenBottomLeftToPanelPosition(positionISX, targetDisplay);
             var delta = pointerState.LastPositionValid ? position - pointerState.LastPosition : Vector2.zero;