WPF - Ensure only single drag operation is emit on DragSourceEndedAt (#4814)

* refactor: ensure only single event is sent to browser host

* fix: remove unnecessary code

* fix: update comments

Author: WilliamTomOBrien

CefSharp.Wpf/ChromiumWebBrowser.cs

@@ -3,6 +3,7 @@
 // Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
 
 using System;
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
@@ -133,7 +134,7 @@ public partial class ChromiumWebBrowser : Control, IRenderWebBrowser, IWpfWebBro
         /// <summary>
         /// Keep the current drag&amp;drop effects to return the appropriate effects on drag over.
         /// </summary>
-        private DragDropEffects currentDragDropEffects;
+        private DragDropEffects? currentDragDropEffects;
         /// <summary>
         /// A flag that indicates whether or not the designer is active
         /// NOTE: Needs to be static for OnApplicationExit
@@ -919,15 +920,25 @@ protected virtual bool StartDragging(IDragData dragData, DragOperationsMask allo
             {
                 if (browser != null)
                 {
-                    //DoDragDrop will fire DragEnter event
+                    // DoDragDrop will fire DragEnter event
                     var result = DragDrop.DoDragDrop(this, dataObject, allowedOps.GetDragEffects());
+                    var dragEffects = currentDragDropEffects.HasValue ? currentDragDropEffects.Value : DragDropEffects.None;
 
-                    //DragData was stored so when DoDragDrop fires DragEnter we reuse a clone of the IDragData provided here
+                    // Ensure the drag and drop operation wasn't cancelled
+                    var finalEffectMask = dragEffects.GetDragOperationsMask() & result.GetDragOperationsMask();
+
+                    // We shouldn't have an instance where finalEffectMask signfies multiple effects, as the operation
+                    // set by UpdateDragCursor should only ever be none, move, copy, or link. However, if it does, we'll
+                    // just default to copy, as that reflects the defaulting behavior of GetDragEffects.
+                    var finalSingleEffect = finalEffectMask.HasFlag(DragOperationsMask.Every) ? DragOperationsMask.Copy : finalEffectMask;
+
+                    // DragData was stored so when DoDragDrop fires DragEnter we reuse a clone of the IDragData provided here
                     currentDragData = null;
+                    currentDragDropEffects = null;
 
-                    //If result == DragDropEffects.None then we'll send DragOperationsMask.None
-                    //effectively cancelling the drag operation
-                    browser.GetHost().DragSourceEndedAt(x, y, result.GetDragOperationsMask());
+                    // If result or the last recorded drag drop effect were DragDropEffects.None
+                    // then we'll send DragOperationsMask.None, effectively cancelling the drag operation
+                    browser.GetHost().DragSourceEndedAt(x, y, finalSingleEffect);
                     browser.GetHost().DragSourceSystemDragEnded();
                 }
             });
@@ -1683,7 +1694,6 @@ private void OnDrop(object sender, DragEventArgs e)
             {
                 var mouseEvent = GetMouseEvent(e);
                 var effect = e.AllowedEffects.GetDragOperationsMask();
-
                 browser.GetHost().DragTargetDragOver(mouseEvent, effect);
                 browser.GetHost().DragTargetDragDrop(mouseEvent);
             }
@@ -1713,7 +1723,7 @@ private void OnDragOver(object sender, DragEventArgs e)
             {
                 browser.GetHost().DragTargetDragOver(GetMouseEvent(e), e.AllowedEffects.GetDragOperationsMask());
             }
-            e.Effects = currentDragDropEffects;
+            e.Effects = currentDragDropEffects.HasValue ? currentDragDropEffects.Value : DragDropEffects.None;
             e.Handled = true;
         }
 
@@ -1731,6 +1741,7 @@ private void OnDragEnter(object sender, DragEventArgs e)
 
                 //DoDragDrop will fire this handler for internally sourced Drag/Drop operations
                 //we use the existing IDragData (cloned copy)
+
                 var dragData = currentDragData ?? e.GetDragData();
 
                 browser.GetHost().DragTargetDragEnter(dragData, mouseEvent, effect);
@@ -2323,7 +2334,7 @@ protected override void OnMouseWheel(MouseWheelEventArgs e)
                     deltaX: isShiftKeyDown ? e.Delta : 0,
                     deltaY: !isShiftKeyDown ? e.Delta : 0,
                     modifiers: modifiers);
-                
+
                 e.Handled = true;
             }
 
@@ -2469,7 +2480,7 @@ private void OnMouseButton(MouseButtonEventArgs e)
                     {
                         clickCount = 1;
                     }
-                    
+
                     MousePositionTransform.TransformMousePoint(ref point);
                     browser.GetHost().SendMouseClickEvent((int)point.X, (int)point.Y, (MouseButtonType)e.ChangedButton, mouseUp, clickCount, modifiers);
                 }