[release/10.0] Always create new small icons for forms (#13984)

Backport of #13983 to release/10.0

/cc @JeremyKuhne

## Customer Impact

Users setting a Form Icon that matches the current screen small icon
size will have their Icon disposed, causing an `ObjectDisposedException`
when using the original Icon. There is no easy workaround.

## Testing

Manual testing with user scenario. Regression test.

## Risk

Low, now always creating a new Icon instance.

Author: JeremyKuhne

src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/ScaleHelper.cs

@@ -424,12 +424,12 @@ internal static HighDpiMode GetThreadHighDpiMode()
     /// <summary>
     ///  Get X, Y metrics at DPI, IF icon is not already that size, create and return a new one.
     /// </summary>
-    internal static Icon ScaleSmallIconToDpi(Icon icon, int dpi)
+    internal static Icon ScaleSmallIconToDpi(Icon icon, int dpi, bool alwaysCreateNew = false)
     {
         int width = PInvoke.GetCurrentSystemMetrics(SYSTEM_METRICS_INDEX.SM_CXSMICON, (uint)dpi);
         int height = PInvoke.GetCurrentSystemMetrics(SYSTEM_METRICS_INDEX.SM_CYSMICON, (uint)dpi);
 
-        return (icon.Width == width && icon.Height == height) ? icon : new(icon, width, height);
+        return (icon.Width == width && icon.Height == height && !alwaysCreateNew) ? icon : new(icon, width, height);
     }
 
     /// <summary>

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

@@ -6453,11 +6453,14 @@ private unsafe void UpdateWindowIcon(bool redrawFrame, int dpi = 0)
 
             if (icon is not null)
             {
-                Icon? oldSmallIcon = _smallIcon;
-
                 try
                 {
-                    _smallIcon = ScaleHelper.ScaleSmallIconToDpi(icon, dpi);
+                    // As we dispose the _smallIcon in multiple places, we'd need to track when we've
+                    // actually created a scaled icon to know when we should free it selectively (so we
+                    // don't free user provided Icons. For now, just always create a new scaled icon.
+                    _smallIcon?.Dispose();
+                    _smallIcon = null;
+                    _smallIcon = ScaleHelper.ScaleSmallIconToDpi(icon, dpi, alwaysCreateNew: true);
                 }
                 catch
                 {
@@ -6466,10 +6469,6 @@ private unsafe void UpdateWindowIcon(bool redrawFrame, int dpi = 0)
                 if (_smallIcon is not null)
                 {
                     PInvokeCore.SendMessage(this, PInvokeCore.WM_SETICON, (WPARAM)PInvoke.ICON_SMALL, (LPARAM)_smallIcon.Handle);
-                    if (oldSmallIcon is not null && oldSmallIcon.Handle != _smallIcon.Handle)
-                    {
-                        oldSmallIcon.Dispose();
-                    }
                 }
 
                 PInvokeCore.SendMessage(this, PInvokeCore.WM_SETICON, (WPARAM)PInvoke.ICON_BIG, (LPARAM)icon.Handle);

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

@@ -2748,6 +2748,21 @@ public void Form_DoesNot_PreventShutDown()
         Assert.True((BOOL)message.ResultInternal);
     }
 
+    [WinFormsFact]
+    public void Form_DoesNot_DisposeUserIcon()
+    {
+        // https://github.com/dotnet/winforms/issues/13963
+        using Form form = new();
+        int dpi = form.DeviceDpi;
+        using Icon icon = new(typeof(Form), "wfc");
+        int smallDpi = PInvoke.GetCurrentSystemMetrics(SYSTEM_METRICS_INDEX.SM_CXSMICON, (uint)dpi);
+        using Icon smallIcon = new(icon, new Size(smallDpi, smallDpi));
+        form.Icon = smallIcon;
+        form.Show();
+        form.Close();
+        smallIcon.Handle.Should().NotBe(0);
+    }
+
     public partial class ParentedForm : Form
     {
         private ParentingForm _parentForm;