Fix setting VisualStylesMode to Classic in the Properties window

[LeafShi1]

Fixes #15012

Root Cause

The VisualStylesMode setter removed the local value when it matched the parent’s resolved mode. In the designer, setting Classic matched the ambient parent value, so the override was removed and the property reverted to Inherit.

Proposed changes

• Only remove the local override when VisualStylesMode.Inherit is explicitly assigned. Preserve explicitly assigned values even when they currently match the parent.

Customer Impact

• Customers can configure VisualStylesMode as Classic through the Visual Studio Properties window. Explicit values also remain stable if the parent’s visual styles mode later changes.

Regression?

• No

Risk

• Minimal

Screenshots

Before

Selecting Classic for Form.VisualStylesMode in the Properties window immediately reverted the property to Inherit and did not serialize the selection.

BeforeChanges.mp4

After

Selecting Classic preserves and serializes the explicit value. Selecting Inherit continues to remove the local override as expected.

AfterChanges.mp4

Test methodology

• Unit test and manual test

Test environment(s)

• .net 11.0.0-rc.1.26424.125

Microsoft Reviewers: Open in CodeFlow

[Copilot]

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Pull request overview

This PR adjusts Control.VisualStylesMode’s ambient/local override behavior so that explicitly assigning a value (e.g., Classic) remains a local override even when it matches the current parent-resolved value, fixing WinForms Designer property-grid reversion/serialization for Classic.

Changes:

• Update Control.VisualStylesMode setter to only clear the local value when VisualStylesMode.Inherit is explicitly assigned.
• Add a unit test that uses PropertyDescriptor.SetValue to validate that setting a value matching the parent preserves ShouldSerializeValue.

File summaries

File	Description
src/System.Windows.Forms/System/Windows/Forms/Control.cs	Changes the VisualStylesMode setter’s logic for when to remove the local override.
src/test/unit/System.Windows.Forms/System/Windows/Forms/ControlTests.VisualStylesMode.cs	Adds a regression test covering property-grid style setting when the value matches the parent.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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