Fix GroupBox Standard and Popup layout overlap when switching to NET11 visual styles (#14891)

## 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:

- Preserve Classic layout metrics for Standard and Popup GroupBoxes in NET11 mode.
- Preserve caption font settings while respecting system text scaling.
- Keep the modern Popup appearance with an accent-colored header and border.
- Leave Flat GroupBox behavior unchanged.
- Add regression tests and update documentation.

Author: LeafShi1

docs/net11-visualstyles-layout-guidance.md

@@ -180,10 +180,11 @@ serialized only when it differs from `Padding.Empty`.
 
 ## Account for GroupBox DisplayRectangle
 
-In modern `FlatStyle.Standard`, the GroupBox caption sits above a borderless rectangular surface. Its
-`DisplayRectangle` starts lower than the classic etched-frame rectangle and reserves more content space above
-than below. The caption aligns with the control Padding. Docked and anchored children are relaid out against the
-new rectangle.
+In modern `FlatStyle.Standard` and `FlatStyle.Popup`, the GroupBox caption sits above a modern surface while
+the `DisplayRectangle` preserves the classic etched-frame rectangle. `FlatStyle.Popup` uses a subtle
+accent-tinted header band and accent border to distinguish the header without changing the content rectangle.
+The caption aligns with the control Padding, and existing child layouts remain stable when switching visual
+styles.
 
 Use the GroupBox as a real layout container:
 
@@ -207,12 +208,10 @@ TableLayoutPanel addressTable = new()
 addressGroup.Controls.Add(addressTable);
 ```
 
-Avoid positioning children with a fixed Y offset derived from `Font.Height`. The renderer owns caption metrics,
-and `DisplayRectangle` is the supported content boundary.
+Avoid positioning children outside `DisplayRectangle`; it remains the supported content boundary.
 
-The modern caption is derived from the ambient font at paint time. Standard and Popup enlarge the caption; Flat
-keeps the ambient size. If the ambient font is regular and a matching installed Semibold family exists, WinForms
-uses that real face. Otherwise the ambient weight is preserved; already styled fonts are never promoted.
+The modern caption is derived from the ambient font at paint time. WinForms preserves the ambient font family,
+size, and style while still following system text scale.
 
 ## React to live text-scale changes
 

src/System.Windows.Forms/System/Windows/Forms/Controls/GroupBox/GroupBox.FlatStyle.Docs.cs

@@ -13,15 +13,14 @@ public partial class GroupBox
     /// <remarks>
     ///  <para>
     ///   In an effective .NET 11-or-later mode, <see cref="FlatStyle.Standard"/> renders a borderless rectangular
-    ///   surface with an enlarged caption, <see cref="FlatStyle.Flat"/> renders a rounded accent outline with an
-    ///   ambient-size inline caption, and <see cref="FlatStyle.Popup"/> renders a Windows-accent header band.
+    ///   surface, <see cref="FlatStyle.Flat"/> renders a rounded accent outline, and <see cref="FlatStyle.Popup"/>
+    ///   renders a rounded accent outline with a subtle accent-tinted header band.
     ///   <see cref="FlatStyle.System"/> remains a native <c>BS_GROUPBOX</c> in every mode.
     ///  </para>
     ///  <para>
-    ///   The modern Standard surface intentionally moves <see cref="Control.DisplayRectangle"/> down and reserves
-    ///   more space above its content than below. Its caption aligns with <see cref="Control.Padding"/>. When the
-    ///   ambient font is regular and a matching installed Semibold family exists, modern captions use that real
-    ///   face; otherwise they preserve the ambient weight. AutoSize layouts remeasure automatically. See the
+    ///   The modern Standard and Popup surfaces preserve the classic <see cref="Control.DisplayRectangle"/> so
+    ///   existing child layouts remain stable when switching visual styles. Modern captions preserve the ambient
+    ///   font family, size, and style while still following system text scale. See the
     ///   <see href="https://github.com/dotnet/winforms/blob/main/docs/net11-visualstyles-layout-guidance.md">
     ///   .NET 11 VisualStyles layout guidance</see>.
     ///  </para>