Issue 2053861 fix combo box flickering issue

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ComboBox/ComboBox.FlatComboAdapter.cs

@@ -60,6 +60,43 @@ public bool IsValid(ComboBox combo)
             return (combo.ClientRectangle == _clientRect && combo.RightToLeft == _origRightToLeft);
         }
 
+        public virtual void DrawPopUpCombo(ComboBox comboBox, Graphics g)
+        {
+            if (comboBox.DropDownStyle == ComboBoxStyle.Simple)
+            {
+                return;
+            }
+
+            bool rightToLeft = comboBox.RightToLeft == RightToLeft.Yes;
+
+            // Draw a dark border around everything if we're in popup mode
+            if ((!comboBox.Enabled) || (comboBox.FlatStyle == FlatStyle.Popup))
+            {
+                bool focused = comboBox.ContainsFocus || comboBox.MouseIsOver;
+                Color borderPenColor = GetPopupOuterBorderColor(comboBox, focused);
+
+                using var borderPen = borderPenColor.GetCachedPenScope();
+                Pen innerPen = comboBox.Enabled ? borderPen : SystemPens.Control;
+
+                // Draw a border around the dropdown.
+                if (rightToLeft)
+                {
+                    g.DrawRectangle(
+                        innerPen,
+                        new Rectangle(_outerBorder.X, _outerBorder.Y, _dropDownRect.Width + 1, _outerBorder.Height));
+                }
+                else
+                {
+                    g.DrawRectangle(
+                        innerPen,
+                        new Rectangle(_dropDownRect.X, _outerBorder.Y, _outerBorder.Right - _dropDownRect.X, _outerBorder.Height));
+                }
+
+                // Draw a border around the whole comboBox.
+                g.DrawRectangle(borderPen, _outerBorder);
+            }
+        }
+
         /// <summary>
         ///  Paints over the edges of the combo box to make it appear flat.
         /// </summary>
@@ -107,33 +144,6 @@ public virtual void DrawFlatCombo(ComboBox comboBox, Graphics g)
             using var innerBorderPen = innerBorderColor.GetCachedPenScope();
             g.DrawRectangle(innerBorderPen, _innerBorder);
             g.DrawRectangle(innerBorderPen, _innerInnerBorder);
-
-            // Draw a dark border around everything if we're in popup mode
-            if ((!comboBox.Enabled) || (comboBox.FlatStyle == FlatStyle.Popup))
-            {
-                bool focused = comboBox.ContainsFocus || comboBox.MouseIsOver;
-                Color borderPenColor = GetPopupOuterBorderColor(comboBox, focused);
-
-                using var borderPen = borderPenColor.GetCachedPenScope();
-                Pen innerPen = comboBox.Enabled ? borderPen : SystemPens.Control;
-
-                // Around the dropdown
-                if (rightToLeft)
-                {
-                    g.DrawRectangle(
-                        innerPen,
-                        new Rectangle(_outerBorder.X, _outerBorder.Y, _dropDownRect.Width + 1, _outerBorder.Height));
-                }
-                else
-                {
-                    g.DrawRectangle(
-                        innerPen,
-                        new Rectangle(_dropDownRect.X, _outerBorder.Y, _outerBorder.Right - _dropDownRect.X, _outerBorder.Height));
-                }
-
-                // Around the whole combobox.
-                g.DrawRectangle(borderPen, _outerBorder);
-            }
         }
 
         /// <summary>

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ComboBox/ComboBox.cs

@@ -3885,7 +3885,14 @@ protected override unsafe void WndProc(ref Message m)
                     }
 
                     using Graphics g = Graphics.FromHdcInternal((IntPtr)dc);
-                    FlatComboBoxAdapter.DrawFlatCombo(this, g);
+                    if ((!Enabled || FlatStyle == FlatStyle.Popup) && MouseIsOver)
+                    {
+                        FlatComboBoxAdapter.DrawPopUpCombo(this, g);
+                    }
+                    else
+                    {
+                        FlatComboBoxAdapter.DrawFlatCombo(this, g);
+                    }
 
                     return;
                 }