[HDPI] ToolStrip overflow menu images appear blurry at high DPI while main ToolStrip images remain sharp

[Olina-Zhang]

.NET version

.NET 11 Preview 6 build: 26304.117

Did it work in .NET Framework?

Not tested/verified

Did it work in any of the earlier releases of .NET Core or .NET 5+?

Repro on .NET 8.0/9.0 and 10.0

Issue description

When a ToolStrip contains enough items to trigger the overflow button, the standard images displayed in the overflow dropdown menu appear noticeably blurred under high DPI settings.
The same images render sharply in the main ToolStrip area.
The issue reproduces using the built-in Insert Standard Items feature and does not require any custom images, ImageLists, or custom renderers.

Steps to reproduce

1. Create a WinForms application.
2. Add a ToolStrip.
3. Use Insert Standard Items to populate the ToolStrip.
4. Add enough items so that the overflow button appears.
5. Run the application with Windows display scaling set to 200%.
6. Open the overflow menu.
   Result: Observe that the images in the overflow menu are noticeably blurrier than the same images displayed in the main ToolStrip.

[LeafShi1]

The root cause of this issue lies in the inconsistent behavior of the WinForms Designer when serializing the Image property of standard ToolStrip items. Specifically, when the overflow menu has not been expanded, the Designer generates a bitmap that has been incorrectly scaled or downsampled and writes it to the .resx file; however, after the overflow menu has been expanded, it generates a bitmap with the correct dimensions. Because the underlying data in the .resx file differs, the runtime display varies between blurry and sharp. Consequently, this is a design-time resource generation (serialization) issue rather than a runtime rendering problem.

@KlausLoeffelmann Are we considering fixing this issue in the Designer repo?

Recording.2026-06-15.122056.mp4