Fixed issue with the combo box popup not being displayed correctly multiple monitor setup.

The screen class is a port from WinForms and updated to C# 7 and be WPF friendly.

Author: Keboo

MaterialDesignThemes.Wpf/ComboBoxPopup.cs

@@ -1,12 +1,10 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
-using System.Diagnostics;
 using System.Linq;
 using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Controls.Primitives;
-using System.Windows.Data;
 using System.Windows.Media;
 
 namespace MaterialDesignThemes.Wpf
@@ -240,7 +238,7 @@ private CustomPopupPlacement[] ComboBoxCustomPopupPlacementCallback(
             if (ClassicMode 
                 || data.LocationX + data.PopupSize.Width - data.RealOffsetX > data.ScreenWidth
                 || data.LocationX - data.RealOffsetX < 0
-                || (!preferUpIfSafe && (data.LocationY - Math.Abs(data.NewDownY) < 0)))
+                || !preferUpIfSafe && data.LocationY - Math.Abs(data.NewDownY) < 0)
             {
                 SetCurrentValue(PopupPlacementProperty, ComboBoxPopupPlacement.Classic);
                 return new[] { GetClassicPopupPlacement(this, data) };
@@ -273,11 +271,13 @@ private PositioningData GetPositioningData(IEnumerable<DependencyObject> visualA
             var mainVisual = visualAncestry.OfType<Visual>().LastOrDefault();
             if (mainVisual == null) throw new ArgumentException($"{nameof(visualAncestry)} must contains unless one {nameof(Visual)} control inside.");
 
-            var screenWidth = (int)DpiHelper.TransformToDeviceX(mainVisual, SystemParameters.PrimaryScreenWidth);
-            var screenHeight = (int)DpiHelper.TransformToDeviceY(mainVisual, SystemParameters.PrimaryScreenHeight);
+            var screen = Screen.FromPoint(locationFromScreen);
+            var screenWidth = (int)DpiHelper.TransformToDeviceX(mainVisual, (int)screen.Bounds.Width);
+            var screenHeight = (int)DpiHelper.TransformToDeviceY(mainVisual, (int)screen.Bounds.Height);
 
-            var locationX = (int)locationFromScreen.X % screenWidth;
-            var locationY = (int)locationFromScreen.Y % screenHeight;
+            //Adjust the location to be in terms of the current screen
+            var locationX = (int)locationFromScreen.X % screenWidth - screen.Bounds.X;
+            var locationY = (int)locationFromScreen.Y % screenHeight - screen.Bounds.Y;
 
             var upVerticalOffsetIndepent = DpiHelper.TransformToDeviceY(mainVisual, UpVerticalOffset);
             var newUpY = upVerticalOffsetIndepent - popupSize.Height + targetSize.Height;

MaterialDesignThemes.Wpf/MaterialDesignThemes.Wpf.csproj

@@ -324,6 +324,7 @@
     <Compile Include="Palette.cs" />
     <Compile Include="Plane3D.cs" />
     <Compile Include="ScaleHost.cs" />
+    <Compile Include="Screen.cs" />
     <Compile Include="Snackbar.cs" />
     <Compile Include="SnackbarMessage.cs" />
     <Compile Include="SnackbarMessageEventArgs.cs" />