Fix issue 14772: Button hosted on a TabPage does not use the Windows accent-colored focus indicator with VisualStylesMode = NET11

[SimonZhao888]

Fixes #14772

##Root Cause

When setting a value for Control.VisualStylesMode, the system treats "setting the child control to the same value currently held by the parent" as "reverting to ambient (inherited) behavior," thereby clearing any local override. Consequently, a child control that appears to be set to Classic actually becomes Inherit; if the parent container later switches to Net11, the child control passively inherits that setting and enables the accent style. This phenomenon is further amplified because the button's implementation includes the ambient background in the modern color-selection logic.

Proposed changes

• Adjusted the assignment semantics for Control.VisualStylesMode: Only the Inherit setting unconditionally clears the local value; the "same as parent" setting clears the local value only when there is an existing local override that is explicitly switched back to "ambient."
• Refined ButtonDarkModeAdapter: Logic for determining accent/default colors and ambient background states now applies only to the _modern (.NET 11) code path, while the Classic path retains its original behavior.
• Added regression tests: Coverage includes key combinations such as .NET 11 containers with Classic child controls, TabPage vs. non-TabPage scenarios, and explicit vs. inherited BackColor settings.

Customer Impact

• Fixed behavior as expected: only Net11 mode controls use Accent; Classic controls (even in Net11 containers) still render as Classic.
• Eliminate the confusion of "the control mode is secretly rewritten by the parent container", and have consistent behavior across containers (Form/TabPage/GroupBox).
• Reduce the risk of UI regressions: Adding new tests continuously prevents the issue from recurring.

Regression?

• No

Risk

• Mini

Screenshots

Before

After

2026-08-06.151846.mp4

14860.mp4

Test methodology

• Manually
• Automated test cases

Test environment(s)

• 11.0.0-preview.7.26381.103

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Fixes WinForms VisualStylesMode inheritance/override semantics so a child control explicitly set to the same mode as its parent does not silently revert to Inherit, which resolves the TabPage-hosted button accent-focus inconsistency in .NET 11 (VisualStylesMode = Net11) scenarios.

Changes:

• Updated Control.VisualStylesMode setter logic to only clear a local override when explicitly set to Inherit, or when intentionally transitioning an existing override back to the ambient (parent) value.
• Refined ButtonDarkModeAdapter background color selection so modern (.NET 11) default-button behavior remains accent-driven while classic paths avoid unintended ambient-color influence.
• Added regression tests covering the new VisualStylesMode semantics and button rendering behavior across TabPage vs non-TabPage hosting and explicit vs inherited BackColor.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/test/unit/System.Windows.Forms/System/Windows/Forms/ControlTests.VisualStylesMode.cs	Adds regression coverage for “first set to ambient value should remain a local override” semantics.
src/test/unit/System.Windows.Forms/System/Windows/Forms/ButtonVisualStylesTests.cs	Adds rendering/regression tests for modern default-button accent behavior and classic-vs-modern BackColor handling.
src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/DarkMode/ButtonDarkModeAdapter.cs	Adjusts back-color selection rules to keep Net11 default buttons accent-driven and prevent classic mode from being influenced by ambient background in modern hierarchies.
src/System.Windows.Forms/System/Windows/Forms/Control.cs	Updates VisualStylesMode override-clearing logic to avoid unintended reversion to Inherit.

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[KlausLoeffelmann]

Hey, could you record a short video, where you - in addition - also Tab through the controls with the Tab key, and see, how the focus indicator looks then through out? Thx!

[SimonZhao888]

Hey, could you record a short video, where you - in addition - also Tab through the controls with the Tab key, and see, how the focus indicator looks then through out? Thx!

Done. I’ve recorded a short video and uploaded it to the "Screenshots" section. The focus indicator displays well when using the Tab key.

Feel free to let me know if you need me to test any other scenarios.

[LeafShi1]

This change makes a Classic button ignore a non-default BackColor inherited from its parent. Before this PR, Control.BackColor—including an ambient value—was passed to the renderer whenever it differed from DefaultBackColor. After this change, ambient colors are accepted only when _modern is true. Consequently, a Classic button on a custom-colored container now renders with the default palette instead of the inherited color. This appears to be a behavioral regression and contradicts the PR description that the Classic path retains its original behavior.