Fix GroupBox Standard and Popup layout overlap when switching to NET11 visual styles

[LeafShi1]

Fixes #14832

Root Cause

GroupBox used larger NET11 caption/header metrics for DisplayRectangle in FlatStyle.Standard and FlatStyle.Popup.

After switching from Classic to NET11, the content rectangle moved, but existing child controls kept their positions. This made controls near the top overlap the GroupBox header/border.

Proposed changes

Update modern GroupBox rendering and layout so that:

• FlatStyle.Standard and FlatStyle.Popup preserve the Classic DisplayRectangle metrics.
• Switching Standard/Popup GroupBoxes from Classic to NET11 no longer requests metric relayout.
• Modern captions preserve the ambient font family, size, and style, while still respecting system text scale.
• FlatStyle.Popup keeps a distinct modern appearance using a subtle accent-tinted header band and accent border.
• FlatStyle.Flat keeps its modern outline metrics, since its layout is tied to the baseline outline renderer.
• Tests are updated to cover Classic-to-NET11 DisplayRectangle stability, docked child stability, caption font preservation, and Popup header rendering.
• GroupBox layout guidance/docs are updated to describe the preserved content rectangle and modern Popup appearance.

Customer Impact

• Apps can enable or switch to NET11 visual styles with less layout disruption. Existing GroupBox child layouts remain stable for Standard and Popup styles.

Regression?

• No

Risk

• Minimal

Screenshots

Before

Switching Classic → NET11 could change the GroupBox content area and cause top child controls to overlap the header/border.

After

Standard and Popup GroupBoxes keep the same content area after switching to NET11, so child controls stay in place and no longer overlap.

Test methodology

• Unit test and manually

Test environment(s)

• .net 11.0.0-rc.1.26411.119

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR adjusts modern GroupBox rendering/layout in .NET 11 visual styles to prevent child-control overlap when switching VisualStylesMode from Classic to Net11, by keeping DisplayRectangle stable for FlatStyle.Standard and FlatStyle.Popup.

Changes:

• Preserve classic DisplayRectangle metrics for modern Standard/Popup and treat Classic→Net11 as repaint-only (no metrics relayout) for those styles.
• Update modern caption behavior to preserve the ambient font family/size/style while still applying system text scale.
• Refresh Popup’s modern appearance (subtle accent-tinted header band + accent border) and update tests/docs accordingly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/test/unit/System.Windows.Forms/System/Windows/Forms/GroupBoxTests.cs	Updates/extends unit tests for mode-switch layout stability, caption font behavior, and Popup rendering expectations.
src/System.Windows.Forms/System/Windows/Forms/Rendering/ModernControlVisualStyles.cs	Removes now-unused GroupBox-related modern constants.
src/System.Windows.Forms/System/Windows/Forms/Controls/GroupBox/GroupBox.Modern.cs	Implements classic-metrics padding for modern Standard/Popup, updates caption font creation, and tweaks Popup header rendering.
src/System.Windows.Forms/System/Windows/Forms/Controls/GroupBox/GroupBox.FlatStyle.Docs.cs	Updates XML docs to reflect preserved DisplayRectangle and new Popup appearance.
src/System.Windows.Forms/System/Windows/Forms/Controls/GroupBox/GroupBox.cs	Refactors DisplayRectangle to use a cached DisplayFontHeight helper and aligns modern/classic calculations.
docs/net11-visualstyles-layout-guidance.md	Updates layout guidance to reflect preserved Standard/Popup content rectangle and revised caption/font guidance.

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