添加项目文件。

Fluent.Ribbon/Fluent.Ribbon/Helpers/PoupHelper.cs

@@ -0,0 +1,52 @@
+namespace Fluent.Helpers
+{
+    using System.Windows;
+    using System.Windows.Controls.Primitives;
+
+    /// <summary>
+    /// Helper class to position <see cref="Popup"/>.
+    /// </summary>
+    public static class PopupHelper
+    {
+        /// <summary>
+        /// Positions <see cref="Popup"/> like <see cref="PlacementMode.Relative"/> would but ignores the value of <see cref="SystemParameters.MenuDropAlignment"/>.
+        /// </summary>
+        public static CustomPopupPlacementCallback SimplePlacementCallback => GetSimplePlacement;
+
+        /// <summary>
+        /// Gets the <see cref="CustomPopupPlacement"/> values for a <see cref="Popup"/> like <see cref="PlacementMode.Relative"/> would but ignores the value of <see cref="SystemParameters.MenuDropAlignment"/>.
+        /// </summary>
+        public static CustomPopupPlacement[] GetSimplePlacement(Size popupSize, Size targetSize, Point offset)
+        {
+            // Create placements which should never cover the target
+            return new[]
+                   {
+                       new CustomPopupPlacement
+                       {
+                           Point = new Point(0, 0),
+                           PrimaryAxis = PopupPrimaryAxis.None
+                       },
+                       new CustomPopupPlacement
+                       {
+                           Point = new Point(-popupSize.Width, 0),
+                           PrimaryAxis = PopupPrimaryAxis.Horizontal
+                       },
+                       new CustomPopupPlacement
+                       {
+                           Point = new Point(0, -popupSize.Height - targetSize.Height),
+                           PrimaryAxis = PopupPrimaryAxis.Vertical
+                       },
+                       new CustomPopupPlacement
+                       {
+                           Point = new Point(-popupSize.Width, -popupSize.Height),
+                           PrimaryAxis = PopupPrimaryAxis.Vertical
+                       },
+                       new CustomPopupPlacement
+                       {
+                           Point = new Point(targetSize.Width, -popupSize.Height),
+                           PrimaryAxis = PopupPrimaryAxis.Horizontal
+                       }
+                   };
+        }
+    }
+}