Fix MaskedTextBox PropertyGrid text editor DPI scaling at runtime

src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/MaskedTextBoxTextEditorDropDown.cs

@@ -5,6 +5,11 @@ namespace System.Windows.Forms.Design;
 
 internal class MaskedTextBoxTextEditorDropDown : UserControl
 {
+    // Logical (96 DPI) constants. The actual device-pixel values are recomputed in
+    // RescaleConstantsForDpi so that PerMonitorV2 DPI changes are honoured.
+    private const int LogicalPadding = 16;
+    private static readonly Drawing.Size s_logicalSize = new(100, 52);
+
     private bool _cancel;
     private readonly MaskedTextBox _cloneMtb;
     private readonly ErrorProvider _errorProvider;
@@ -48,8 +53,11 @@ public MaskedTextBoxTextEditorDropDown(MaskedTextBox maskedTextBox)
         BackColor = Drawing.SystemColors.Control;
         BorderStyle = BorderStyle.FixedSingle;
         Name = "MaskedTextBoxTextEditorDropDown";
-        Padding = new Padding(16);
-        Size = new Drawing.Size(100, 52);
+
+        // Apply the initial DPI-scaled padding/size. Subsequent DPI changes are
+        // picked up automatically through RescaleConstantsForDpi.
+        RescaleConstantsForDpi(DeviceDpi, DeviceDpi);
+
         ((System.ComponentModel.ISupportInitialize)(_errorProvider)).EndInit();
         ResumeLayout(false);
         PerformLayout();
@@ -80,6 +88,16 @@ protected override bool ProcessDialogKey(Keys keyData)
         return base.ProcessDialogKey(keyData);
     }
 
+    protected override void RescaleConstantsForDpi(int deviceDpiOld, int deviceDpiNew)
+    {
+        base.RescaleConstantsForDpi(deviceDpiOld, deviceDpiNew);
+
+        // DeviceDpi has already been updated to deviceDpiNew by the framework before
+        // this call, so LogicalToDeviceUnits returns values scaled to the new DPI.
+        Padding = new Padding(LogicalToDeviceUnits(LogicalPadding));
+        Size = LogicalToDeviceUnits(s_logicalSize);
+    }
+
     private void maskedTextBox_MaskInputRejected(object? sender, MaskInputRejectedEventArgs e)
     {
         _errorProvider.SetError(_cloneMtb, MaskedTextBoxDesigner.GetMaskInputRejectedErrorMessage(e));

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/MaskedTextBoxTextEditorDropDownTests.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Drawing;
+
 namespace System.Windows.Forms.Design.Tests;
 
 public class MaskedTextBoxTextEditorDropDownTests
@@ -54,4 +56,17 @@ public void MaskInputRejected_SetsError_WhenInputRejected()
         dropDownMaskedTextBox.Text = "invalid";
         errorProvider.GetError(dropDownMaskedTextBox).Should().Contain(SR.MaskedTextBoxHintDigitExpected);
     }
+
+    [Fact]
+    public void DropDown_SizeAndPadding_ScalesWithDPI()
+    {
+        using MaskedTextBox maskedTextBox = new();
+        using MaskedTextBoxTextEditorDropDown dropDown = new(maskedTextBox);
+
+        // Simulate a DPI change and verify that the dropdown recomputes its logical constants.
+        dropDown.TestAccessor.Dynamic.RescaleConstantsForDpi(96, 192);
+
+        dropDown.Size.Should().Be(new Size(100, 52));
+        dropDown.Padding.Should().Be(new Padding(16));
+    }
 }