Workaround to UGUI input issues https://issuetracker.unity3d.com/issues/broken-mouse-pointer-input-coordinates-in-graphicraycaster-for-certain-window-sizes-in-build-when-run-in-windowed-mode

MultipleDisplayUtilities.cs

using UnityEngine.EventSystems;

namespace UnityEngine.UI
{
    internal static class MultipleDisplayUtilities
    {
        /// <summary>
        /// Converts the current drag position into a relative position for the display.
        /// </summary>
        /// <param name="eventData"></param>
        /// <param name="position"></param>
        /// <returns>Returns true except when the drag operation is not on the same display as it originated</returns>
        public static bool GetRelativeMousePositionForDrag(PointerEventData eventData, ref Vector2 position)
        {
            #if UNITY_EDITOR
            position = eventData.position;
            #else
            int pressDisplayIndex = eventData.pointerPressRaycast.displayIndex;
            var relativePosition = RelativeMouseAtScaled(eventData.position);
            int currentDisplayIndex = (int)relativePosition.z;

            // Discard events on a different display.
            if (currentDisplayIndex != pressDisplayIndex)
                return false;

            // If we are not on the main display then we must use the relative position.
            position = pressDisplayIndex != 0 ? (Vector2)relativePosition : eventData.position;
            #endif
            return true;
        }

        /// <summary>
        /// A version of Display.RelativeMouseAt that scales the position when the main display has a different rendering resolution to the system resolution.
        /// By default, the mouse position is relative to the main render area, we need to adjust this so it is relative to the system resolution
        /// in order to correctly determine the position on other displays.
        /// </summary>
        /// <returns></returns>
        public static Vector3 RelativeMouseAtScaled(Vector2 position)
        {
            #if !UNITY_EDITOR && !UNITY_WSA
            // If the main display is now the same resolution as the system then we need to scale the mouse position. (case 1141732)
            if (Display.main.renderingWidth != Display.main.systemWidth || Display.main.renderingHeight != Display.main.systemHeight)
            {
                // The system will add padding when in full-screen and using a non-native aspect ratio. (case UUM-7893)
                // For example Rendering 1920x1080 with a systeem resolution of 3440x1440 would create black bars on each side that are 330 pixels wide.
                // we need to account for this or it will offset our coordinates when we are not on the main display.
                var systemAspectRatio = Display.main.systemWidth / (float)Display.main.systemHeight;

                var sizePlusPadding = new Vector2(Display.main.renderingWidth, Display.main.renderingHeight);
                var padding = Vector2.zero;
                if (Screen.fullScreen)
                {
                    var aspectRatio = Screen.width / (float)Screen.height;
                    if (Display.main.systemHeight * aspectRatio < Display.main.systemWidth)
                    {
                        // Horizontal padding
                        sizePlusPadding.x = Display.main.renderingHeight * systemAspectRatio;
                        padding.x = (sizePlusPadding.x - Display.main.renderingWidth) * 0.5f;
                    }
                    else
                    {
                        // Vertical padding
                        sizePlusPadding.y = Display.main.renderingWidth / systemAspectRatio;
                        padding.y = (sizePlusPadding.y - Display.main.renderingHeight) * 0.5f;
                    }
                }

                var sizePlusPositivePadding = sizePlusPadding - padding;

                // If we are not inside of the main display then we must adjust the mouse position so it is scaled by
                // the main display and adjusted for any padding that may have been added due to different aspect ratios.
                if (position.y < -padding.y || position.y > sizePlusPositivePadding.y ||
                     position.x < -padding.x || position.x > sizePlusPositivePadding.x)
                {
                    var adjustedPosition = position;

                    if (!Screen.fullScreen)
                    {
                        // When in windowed mode, the window will be centered with the 0,0 coordinate at the top left, we need to adjust so it is relative to the screen instead.
                        adjustedPosition.x -= (Display.main.renderingWidth - Display.main.systemWidth) * 0.5f;
                        adjustedPosition.y -= (Display.main.renderingHeight - Display.main.systemHeight) * 0.5f;
                    }
                    else
                    {
                        // Scale the mouse position to account for the black bars when in a non-native aspect ratio.
                        adjustedPosition += padding;
                        adjustedPosition.x *= Display.main.systemWidth / sizePlusPadding.x;
                        adjustedPosition.y *= Display.main.systemHeight / sizePlusPadding.y;
                    }

                    var relativePos = Display.RelativeMouseAt(adjustedPosition);

                    // If we are not on the main display then return the adjusted position.
                    if (relativePos.z != 0)
                        return relativePos;
                }

                // We are using the main display.
                return new Vector3(position.x, position.y, 0);
            }
            #endif
            return Display.RelativeMouseAt(position);
        }
    }
}