Add AppContext switch to mirror Owner.TopMost for modal dialogs (restore .NET Framework‑like behavior)

src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs

@@ -26,6 +26,7 @@ internal static partial class LocalAppContextSwitches
     internal const string EnableMsoComponentManagerSwitchName = "Switch.System.Windows.Forms.EnableMsoComponentManager";
     internal const string TreeNodeCollectionAddRangeRespectsSortOrderSwitchName = "System.Windows.Forms.TreeNodeCollectionAddRangeRespectsSortOrder";
     internal const string MoveTreeViewTextLocationOnePixelSwitchName = "System.Windows.Forms.TreeView.MoveTreeViewTextLocationOnePixel";
+    internal const string MirrorTopMostForModalDialogsSwitchName = "System.Windows.Forms.MirrorTopMostForModalDialogs";
 
     private static int s_scaleTopLevelFormMinMaxSizeForDpi;
     private static int s_anchorLayoutV2;
@@ -37,6 +38,7 @@ internal static partial class LocalAppContextSwitches
     private static int s_noClientNotifications;
     private static int s_enableMsoComponentManager;
     private static int s_treeNodeCollectionAddRangeRespectsSortOrder;
+    private static int s_mirrorTopMostForModalDialogs;
 
     private static int s_moveTreeViewTextLocationOnePixel;
 
@@ -222,6 +224,18 @@ public static bool TreeNodeCollectionAddRangeRespectsSortOrder
         get => GetCachedSwitchValue(TreeNodeCollectionAddRangeRespectsSortOrderSwitchName, ref s_treeNodeCollectionAddRangeRespectsSortOrder);
     }
 
+    /// <summary>
+    ///  When true, ShowDialog will temporarily set the dialog as TopMost if its owner is TopMost.
+    ///  This prevents the dialog from being covered by the owner during the modal session.
+    ///  The original TopMost state is restored after closing.
+    ///  Default is false. Set to true for .NET Framework-compatible behavior.
+    /// </summary>
+    public static bool MirrorTopMostForModalDialogs
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        get => GetCachedSwitchValue(MirrorTopMostForModalDialogsSwitchName, ref s_mirrorTopMostForModalDialogs);
+    }
+
     /// <summary>
     ///  Indicates whether to move the text position of a TreeView node one pixel
     ///  to the right relative to the upper-left corner of the TreeView control.

src/System.Windows.Forms/System/Windows/Forms/Form.cs

@@ -5718,6 +5718,9 @@ public DialogResult ShowDialog(IWin32Window? owner)
 
         Form? oldOwner = OwnerInternal;
 
+        bool originalTopMost = TopMost;
+        bool mirrored = false;
+
         try
         {
             SetState(States.Modal, true);
@@ -5755,6 +5758,12 @@ public DialogResult ShowDialog(IWin32Window? owner)
                 if (owner is Form form && owner != oldOwner)
                 {
                     Owner = form;
+
+                    if (AppContextSwitches.MirrorTopMostForModalDialogs && TopLevel && !IsMdiChild && Owner.TopMost && !TopMost)
+                    {
+                        TopMost = true;
+                        mirrored = true;
+                    }
                 }
                 else
                 {
@@ -5814,6 +5823,11 @@ public DialogResult ShowDialog(IWin32Window? owner)
             Owner = oldOwner;
             Properties.RemoveValue(s_propDialogOwner);
             GC.KeepAlive(ownerHwnd.Wrapper);
+
+            if (mirrored)
+            {
+                TopMost = originalTopMost;
+            }
         }
 
         return DialogResult;