Backport some WCT DataGrid fixes

src/Avalonia.Controls.DataGrid/DataGridColumns.cs

@@ -12,6 +12,7 @@ using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.Diagnostics;
 using System.Reflection;
+using Avalonia.Layout;
 
 namespace Avalonia.Controls
 {
@@ -489,7 +490,7 @@ namespace Avalonia.Controls
         {
             DataGridFillerColumn fillerColumn = ColumnsInternal.FillerColumn;
             double totalColumnsWidth = ColumnsInternal.VisibleEdgedColumnsWidth;
-            if (finalWidth > totalColumnsWidth)
+            if (finalWidth - totalColumnsWidth > LayoutHelper.LayoutEpsilon)
             {
                 fillerColumn.FillerWidth = finalWidth - totalColumnsWidth;
             }
@@ -971,6 +972,12 @@ namespace Avalonia.Controls
                         {
                             cx += _negHorizontalOffset;
                             _horizontalOffset -= _negHorizontalOffset;
+                            if (_horizontalOffset < LayoutHelper.LayoutEpsilon)
+                            {
+                                // Snap to zero to avoid trying to partially scroll in first scrolled off column below
+                                _horizontalOffset = 0;
+                            }
+
                             _negHorizontalOffset = 0;
                         }
                         else
@@ -979,6 +986,11 @@ namespace Avalonia.Controls
                             _negHorizontalOffset -= displayWidth - cx;
                             cx = displayWidth;
                         }
+
+                        // Make sure the HorizontalAdjustment is not greater than the new HorizontalOffset
+                        // since it would cause an assertion failure in DataGridCellsPresenter.ShouldDisplayCell
+                        // called by DataGridCellsPresenter.MeasureOverride.
+                        HorizontalAdjustment = Math.Min(HorizontalAdjustment, _horizontalOffset);
                     }
                     // second try to scroll entire columns
                     if (cx < displayWidth && _horizontalOffset > 0)

src/Avalonia.Controls.DataGrid/DataGridRows.cs

@@ -1589,6 +1589,23 @@ namespace Avalonia.Controls
             CorrectSlotsAfterDeletion(slot, isRow);
 
             OnRemovedElement(slot, item);
+            
+            // Synchronize CurrentCellCoordinates, CurrentColumn, CurrentColumnIndex, CurrentItem
+            // and CurrentSlot with the currently edited cell, since OnRemovingElement called
+            // SetCurrentCellCore(-1, -1) to temporarily reset the current cell.
+            if (_temporarilyResetCurrentCell &&
+                _editingColumnIndex != -1 &&
+                _previousCurrentItem != null &&
+                EditingRow != null &&
+                EditingRow.Slot != -1)
+            {
+                ProcessSelectionAndCurrency(
+                    columnIndex: _editingColumnIndex,
+                    item: _previousCurrentItem,
+                    backupSlot: this.EditingRow.Slot,
+                    action: DataGridSelectionAction.None,
+                    scrollIntoView: false);
+            }
         }
 
         private void RemoveNonDisplayedRows(int newFirstDisplayedSlot, int newLastDisplayedSlot)