Fix ItemHeight of ListBox is not scaled on high DPI in OwnerDrawFixed mode

src/System.Windows.Forms/System/Windows/Forms/Controls/ListBoxes/ListBox.cs

@@ -2071,6 +2071,15 @@ protected override void ScaleControl(SizeF factor, BoundsSpecified specified)
             UpdateFontCache();
         }
 
+        // In OwnerDrawFixed the native list box doesn't rescale the fixed item height when the control
+        // autoscales, so scale it here (clamped to [1, 255]; height-axis guard avoids a double-apply). #6382
+        // Round away from zero: banker's rounding would round .5 midpoints down (15 * 1.5 -> 22), which
+        // re-introduces the cramped rows this fixes.
+        if (_drawMode == DrawMode.OwnerDrawFixed && factor.Height != 1F && (specified & BoundsSpecified.Height) != 0)
+        {
+            ItemHeight = Math.Clamp((int)Math.Round(_itemHeight * factor.Height, MidpointRounding.AwayFromZero), 1, 255);
+        }
+
         base.ScaleControl(factor, specified);
     }
 

src/test/unit/System.Windows.Forms/System/Windows/Forms/ListBoxTests.cs

@@ -1845,6 +1845,76 @@ public void ListBox_ItemHeight_ShouldSerializeValue_Success()
         Assert.False(property.ShouldSerializeValue(control));
     }
 
+    [WinFormsFact]
+    public void ListBox_ScaleControl_OwnerDrawFixed_ScalesItemHeight()
+    {
+        // #6382: an OwnerDrawFixed item height must scale with the control during autoscaling.
+        using SubListBox control = new() { DrawMode = DrawMode.OwnerDrawFixed, ItemHeight = 20 };
+
+        control.ScaleControl(new SizeF(1.5f, 1.5f), BoundsSpecified.All);
+
+        Assert.Equal(30, control.ItemHeight);
+    }
+
+    [WinFormsFact]
+    public void ListBox_ScaleControl_OwnerDrawFixed_WithHandle_UpdatesNativeItemHeight()
+    {
+        using SubListBox control = new() { DrawMode = DrawMode.OwnerDrawFixed, ItemHeight = 20 };
+        Assert.NotEqual(IntPtr.Zero, control.Handle);
+        Assert.Equal(20, (int)PInvokeCore.SendMessage(control, PInvoke.LB_GETITEMHEIGHT));
+
+        control.ScaleControl(new SizeF(2f, 2f), BoundsSpecified.All);
+
+        Assert.Equal(40, control.ItemHeight);
+        Assert.Equal(40, (int)PInvokeCore.SendMessage(control, PInvoke.LB_GETITEMHEIGHT));
+    }
+
+    [WinFormsTheory]
+    [InlineData(25, 38)] // 37.5 rounds up
+    [InlineData(15, 23)] // 22.5 rounds up, not down to 22 as banker's rounding would
+    public void ListBox_ScaleControl_OwnerDrawFixed_RoundsMidpointsAwayFromZero(int itemHeight, int expected)
+    {
+        using SubListBox control = new() { DrawMode = DrawMode.OwnerDrawFixed, ItemHeight = itemHeight };
+
+        control.ScaleControl(new SizeF(1.5f, 1.5f), BoundsSpecified.All);
+
+        Assert.Equal(expected, control.ItemHeight);
+    }
+
+    [WinFormsFact]
+    public void ListBox_ScaleControl_OwnerDrawFixed_HeightExcluded_DoesNotScaleItemHeight()
+    {
+        // A scaling pass that excludes the height axis must leave the item height alone, otherwise the
+        // included/excluded pass pair in ContainerControl.PerformAutoScale would apply the factor twice.
+        using SubListBox control = new() { DrawMode = DrawMode.OwnerDrawFixed, ItemHeight = 25 };
+
+        control.ScaleControl(new SizeF(1.5f, 1.5f), BoundsSpecified.Width);
+
+        Assert.Equal(25, control.ItemHeight);
+    }
+
+    [WinFormsFact]
+    public void ListBox_ScaleControl_OwnerDrawFixed_ClampsItemHeightToMax()
+    {
+        // Scaling must not throw when the result exceeds the 255 maximum; it clamps instead.
+        using SubListBox control = new() { DrawMode = DrawMode.OwnerDrawFixed, ItemHeight = 200 };
+
+        control.ScaleControl(new SizeF(2f, 2f), BoundsSpecified.All);
+
+        Assert.Equal(255, control.ItemHeight);
+    }
+
+    [WinFormsFact]
+    public void ListBox_ScaleControl_Normal_DoesNotChangeItemHeight()
+    {
+        using SubListBox control = new() { DrawMode = DrawMode.Normal };
+        int before = control.ItemHeight;
+
+        control.ScaleControl(new SizeF(1.5f, 1.5f), BoundsSpecified.All);
+
+        Assert.Equal(before, control.ItemHeight);
+    }
+
     public static IEnumerable<object[]> Items_CustomCreateItemCollection_TestData()
     {
         yield return new object[] { null };