FIX: Reset ButtonStates when re-using eventData with SingleUnifiedPointer (#2139)

Author: tspiller

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -12,6 +12,7 @@ however, it has to be formatted properly to pass verification tests.
 
 ### Fixed
 - Fixed an issue where all action maps were enabled initially for project wide actions, which overrode the PlayerInput action map configuration. [ISXB-920](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-920)
+- Fixed an issue where ButtonStates are not fully updated when switching SingleUnifiedPointer. [ISXB-1356](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1356)
 
 ### Changed
 - Changed enum value `Key.IMESelected` to obsolete which was not a real key. Please use the ButtonControl `imeSelected`.

Packages/com.unity.inputsystem/InputSystem/Plugins/UI/InputSystemUIInputModule.cs

@@ -1931,6 +1931,15 @@ private int GetPointerStateIndexFor(InputControl control, bool createIfNotExists
                     // Make sure these don't linger around when we switch to a different kind of pointer.
                     eventData.trackedDeviceOrientation = default;
                     eventData.trackedDevicePosition = default;
+
+                    // We only have a single pointer state and current frame press state values was based on previous eventData.
+                    // Make sure these get updated when we switch.
+                    if (m_PointerBehavior == UIPointerBehavior.SingleUnifiedPointer)
+                    {
+                        pointer.leftButton.OnEndFrame();
+                        pointer.rightButton.OnEndFrame();
+                        pointer.middleButton.OnEndFrame();
+                    }
                 }
 
                 if (pointerType == UIPointerType.Touch)