fix hang when margin is not a multiple of DPI rounding quantum (#5841) (#6004)

Co-authored-by: NSam Bent <sambent@microsoft.com>

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ItemsControl.cs

@@ -3079,6 +3079,38 @@ private bool IsOnCurrentPage(FrameworkElement viewPort, FrameworkElement element
             return ElementViewportPosition.None;
         }
 
+        // this version also returns the element's layout rectangle (in viewport's coordinates).
+        // VirtualizingStackPanel needs this, to determine the element's scroll offset.
+        internal static ElementViewportPosition GetElementViewportPosition(FrameworkElement viewPort,
+            UIElement element,
+            FocusNavigationDirection axis,
+            bool fullyVisible,
+            bool ignorePerpendicularAxis,
+            out Rect elementRect,
+            out Rect layoutRect)
+        {
+            ElementViewportPosition position = GetElementViewportPosition(
+                viewPort,
+                element,
+                axis,
+                fullyVisible,
+                false,
+                out elementRect);
+
+            if (position == ElementViewportPosition.None)
+            {
+                layoutRect = Rect.Empty;
+            }
+            else
+            {
+                Visual parent = VisualTreeHelper.GetParent(element) as Visual;
+                Debug.Assert(element != viewPort && element.IsArrangeValid && parent != null, "GetElementViewportPosition called in unsupported situation");
+                layoutRect = CorrectCatastrophicCancellation(parent.TransformToAncestor(viewPort)).TransformBounds(element.PreviousArrangeRect);
+            }
+
+            return position;
+        }
+
         // in large virtualized hierarchical lists (TreeView or grouping), the transform
         // returned by element.TransformToAncestor(viewport) is vulnerable to catastrophic
         // cancellation.  If element is at the top of the viewport, but embedded in

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/VirtualizingStackPanel.cs

@@ -1189,6 +1189,7 @@ private void ClearAnchorInformation(bool shouldAbort)
                     if (fe.IsVisible)
                     {
                         Rect elementRect;
+                        Rect layoutRect;
 
                         // get the vp-position of the element, ignoring the secondary axis
                         // (DevDiv2 1136036, 1203626 show two different cases why we
@@ -1201,7 +1202,8 @@ private void ClearAnchorInformation(bool shouldAbort)
                             direction,
                             false /*fullyVisible*/,
                             !isVSP45Compat /*ignorePerpendicularAxis*/,
-                            out elementRect);
+                            out elementRect,
+                            out layoutRect);
 
                         if (elementPosition == ElementViewportPosition.PartiallyInViewport ||
                             elementPosition == ElementViewportPosition.CompletelyInViewport)
@@ -1287,20 +1289,35 @@ private void ClearAnchorInformation(bool shouldAbort)
                                 {
                                     if (direction == FocusNavigationDirection.Down)
                                     {
-                                        firstContainerOffsetFromViewport = elementRect.Y;
-                                        if (!isVSP45Compat)
-                                        {
-                                            firstContainerOffsetFromViewport -= fe.Margin.Top;
-                                        }
-                                    }
+                                       if (isVSP45Compat)
+                                       {
+                                           firstContainerOffsetFromViewport = elementRect.Y;
+                                       }
+                                       else
+                                       {
+                                           // include the leading margin in the offset.  Simply subtracting
+                                           // the margin doesn't work when layout rounding is in effect, as
+                                           // we can't deduce how rounding affected the arrangement of the
+                                           // element and its margin.  Instead, just use the layout rect directly.
+                                           firstContainerOffsetFromViewport = layoutRect.Top;
+                                       }
+                                   }
                                     else // (direction == FocusNavigationDirection.Right)
                                     {
-                                        firstContainerOffsetFromViewport = elementRect.X;
-                                        if (!isVSP45Compat)
-                                        {
-                                            firstContainerOffsetFromViewport -= fe.Margin.Left;
-                                        }
-                                    }
+
+                                       if (isVSP45Compat)
+                                       {
+                                           firstContainerOffsetFromViewport = elementRect.X;
+                                       }
+                                       else
+                                       {
+                                           // include the leading margin in the offset.  Simply subtracting
+                                           // the margin doesn't work when layout rounding is in effect, as
+                                           // we can't deduce how rounding affected the arrangement of the
+                                           // element and its margin.  Instead, just use the layout rect directly.
+                                           firstContainerOffsetFromViewport = layoutRect.Left;
+                                       }
+                                   }
                                 }
                                 else if (findTopContainer && isTopContainer)
                                 {
@@ -1931,7 +1948,7 @@ public ScrollViewer ScrollOwner
             }
             set
             {
-                if (_scrollData == null) EnsureScrollData();
+                EnsureScrollData();
                 if (value != _scrollData._scrollOwner)
                 {
                     ResetScrolling(this);
@@ -9510,10 +9527,6 @@ private int ItemCount
         private void EnsureScrollData()
         {
             if (_scrollData == null) { _scrollData = new ScrollData(); }
-            else
-            {
-                Debug.Assert(_scrollData._scrollOwner != null, "Scrolling an unconnected VSP");
-            }
         }
 
         private static void ResetScrolling(VirtualizingStackPanel element)