FIX: Add test and fix for multiple touchscreen input bug.

Assets/Tests/InputSystem/TouchscreenMultiDisplayTests.cs

@@ -0,0 +1,84 @@
+using NUnit.Framework;
+using UnityEngine;
+using UnityEngine.InputSystem;
+using UnityEngine.InputSystem.LowLevel;
+using TouchPhase = UnityEngine.InputSystem.TouchPhase;
+
+// Tests covering touch tracking across multiple physical touchscreen monitors.
+// Regression coverage for IN-108611: touches from one screen incorrectly matching
+// ongoing touches from another screen when both screens report the same touchId.
+[TestFixture]
+internal class TouchscreenMultiDisplayTests : CoreTestsFixture
+{
+    // When two physical touchscreens both have an active touch with the same touchId,
+    // a Move event from screen 2 must update screen 2's touch slot, not screen 1's.
+    //
+    // Failure mode (pre-fix): OnStateEvent matches ongoing touches by touchId alone,
+    // so screen 2's Move event finds screen 1's slot first (both have touchId=1) and
+    // incorrectly updates it, leaving screen 1's position changed and screen 2 stale.
+    [Test]
+    [Category("Devices")]
+    public void Devices_TouchMoveOnSecondDisplay_DoesNotUpdateTouchOnFirstDisplay()
+    {
+        var device = InputSystem.AddDevice<Touchscreen>();
+
+        // Finger down on display 0 (touchId=1).
+        InputSystem.QueueStateEvent(device, new TouchState
+        {
+            phase = TouchPhase.Began,
+            touchId = 1,
+            position = new Vector2(100, 100),
+            displayIndex = 0,
+        });
+
+        // Finger down on display 1 — same touchId, different screen.
+        InputSystem.QueueStateEvent(device, new TouchState
+        {
+            phase = TouchPhase.Began,
+            touchId = 1,
+            position = new Vector2(200, 200),
+            displayIndex = 1,
+        });
+
+        InputSystem.Update();
+
+        // Both touches should be allocated to separate slots.
+        Assert.That(device.touches[0].phase.ReadValue(), Is.EqualTo(TouchPhase.Began));
+        Assert.That(device.touches[1].phase.ReadValue(), Is.EqualTo(TouchPhase.Began));
+
+        var display0SlotIndex = -1;
+        var display1SlotIndex = -1;
+        for (var i = 0; i < 2; i++)
+        {
+            var displayIdx = device.touches[i].displayIndex.ReadValue();
+            if (displayIdx == 0) display0SlotIndex = i;
+            else if (displayIdx == 1) display1SlotIndex = i;
+        }
+
+        Assert.That(display0SlotIndex, Is.Not.EqualTo(-1), "No touch slot found for display 0");
+        Assert.That(display1SlotIndex, Is.Not.EqualTo(-1), "No touch slot found for display 1");
+
+        var display0PositionBefore = device.touches[display0SlotIndex].position.ReadValue();
+
+        // Swipe on display 1 (same touchId=1 as display 0's held touch).
+        InputSystem.QueueStateEvent(device, new TouchState
+        {
+            phase = TouchPhase.Moved,
+            touchId = 1,
+            position = new Vector2(300, 300),
+            displayIndex = 1,
+        });
+
+        InputSystem.Update();
+
+        // Display 0's touch must be unchanged.
+        Assert.That(device.touches[display0SlotIndex].position.ReadValue(), Is.EqualTo(display0PositionBefore),
+            "Touch on display 0 was incorrectly updated by a Move event from display 1");
+
+        // Display 1's touch must reflect the new position.
+        Assert.That(device.touches[display1SlotIndex].position.ReadValue(), Is.EqualTo(new Vector2(300, 300)),
+            "Touch on display 1 was not updated by its own Move event");
+
+        Assert.That(device.touches[display1SlotIndex].phase.ReadValue(), Is.EqualTo(TouchPhase.Moved));
+    }
+}

Packages/com.unity.inputsystem/InputSystem/Runtime/Devices/Touchscreen.cs

@@ -694,7 +694,7 @@ protected override void FinishSetup()
                 var touchId = newTouchState.touchId;
                 for (var i = 0; i < touchControlCount; ++i)
                 {
-                    if (currentTouchState[i].touchId == touchId)
+                    if (currentTouchState[i].touchId == touchId && currentTouchState[i].displayIndex == newTouchState.displayIndex)
                     {
                         // Preserve primary touch state.
                         var isPrimaryTouch = currentTouchState[i].isPrimaryTouch;
@@ -915,7 +915,7 @@ unsafe bool IInputStateCallbackReceiver.GetStateOffsetForEvent(InputControl cont
                 for (var i = 0; i < touchControlCount; ++i)
                 {
                     var touch = &currentTouchState[i];
-                    if (touch->touchId == eventTouchId || (!touch->isInProgress && eventTouchPhase.IsActive()))
+                    if ((touch->touchId == eventTouchId && touch->displayIndex == eventTouchState->displayIndex) || (!touch->isInProgress && eventTouchPhase.IsActive()))
                     {
                         offset = primaryTouch.m_StateBlock.byteOffset + primaryTouch.m_StateBlock.alignedSizeInBytes - m_StateBlock.byteOffset +
                             (uint)(i * UnsafeUtility.SizeOf<TouchState>());
@@ -1002,7 +1002,7 @@ internal static unsafe bool MergeForward(InputEventPtr currentEventPtr, InputEve
             var currentState = (TouchState*)currentEvent->state;
             var nextState = (TouchState*)nextEvent->state;
 
-            if (currentState->touchId != nextState->touchId || currentState->phaseId != nextState->phaseId || currentState->flags != nextState->flags)
+            if (currentState->touchId != nextState->touchId || currentState->phaseId != nextState->phaseId || currentState->flags != nextState->flags || currentState->displayIndex != nextState->displayIndex)
                 return false;
 
             nextState->delta += currentState->delta;

Packages/com.unity.inputsystem/InputSystem/Runtime/Plugins/EnhancedTouch/TouchSimulation.cs

@@ -253,6 +253,9 @@ protected void OnEnable()
             if (m_TouchIds == null)
                 m_TouchIds = new int[simulatedTouchscreen.touches.Count];
 
+            if (m_TouchDisplayIndices == null)
+                m_TouchDisplayIndices = new byte[simulatedTouchscreen.touches.Count];
+
             foreach (var device in InputSystem.devices)
                 OnDeviceChange(device, InputDeviceChange.Added);
 
@@ -306,10 +309,12 @@ private unsafe void UpdateTouch(int touchIndex, int pointerIndex, TouchPhase pha
                 touch.startPosition = position;
                 touch.touchId = ++m_LastTouchId;
                 m_TouchIds[touchIndex] = m_LastTouchId;
+                m_TouchDisplayIndices[touchIndex] = displayIndex;
             }
             else
             {
                 touch.touchId = m_TouchIds[touchIndex];
+                touch.displayIndex = m_TouchDisplayIndices[touchIndex];
             }
 
             //NOTE: Processing these events still happen in the current frame.
@@ -327,6 +332,7 @@ private unsafe void UpdateTouch(int touchIndex, int pointerIndex, TouchPhase pha
         [NonSerialized] private int[] m_CurrentDisplayIndices;
         [NonSerialized] private ButtonControl[] m_Touches;
         [NonSerialized] private int[] m_TouchIds;
+        [NonSerialized] private byte[] m_TouchDisplayIndices;
 
         [NonSerialized] private int m_LastTouchId;
         [NonSerialized] private Action<InputDevice, InputDeviceChange> m_OnDeviceChange;