WinForms Designer Regression: Cannot Select EnableDesignMode-Exposed Panel by Click in .NET 9

### Environment

### Summary

**In WinForms .NET 9:** (Version 17.14.10)
When building a custom UserControl that exposes an internal panel with 'EnableDesignMode', it is not possible to select this internal panel by clicking in the designer. All drag-and-drop operations add controls to the root UserControl, not the design-surface-exposed panel, unless the Document Outline is used.

**This worked as expected in .NET Framework 4.8.1,** where clicking the internal panel selected it for control drops.
**In .NET 9, this is broken** and makes composite control design much harder.

---

### Minimal Repro

### Steps to Reproduce

1. Add the following minimal UserControl and designer to a .NET WinForms project:

```csharp
using System.ComponentModel;
using System.Windows.Forms;
using System.Windows.Forms.Design;

[Designer(typeof(DemoPanelDesigner))]
public class DemoPanel : UserControl
{
private readonly Panel _contentPanel = new Panel();

public DemoPanel()
{
_contentPanel.Dock = DockStyle.Fill;
this.Controls.Add(_contentPanel);
}

[Browsable(true)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
public Panel ContentPanel => _contentPanel;
}

public class DemoPanelDesigner : ParentControlDesigner
{
public override void Initialize(IComponent component)
{
base.Initialize(component);

if (component is DemoPanel panel)
EnableDesignMode(panel.ContentPanel, nameof(DemoPanel.ContentPanel));
}
}
```

2. Build the project, open a Form in the designer, and add 'DemoPanel' to it.
3. Try to drag a Button (or any control) and drop it inside the 'DemoPanel' surface.
4. Observe the generated code:

```csharp
// Result:
demoPanel1.Controls.Add(this.button1); // added to UserControl.Controls, not ContentPanel.Controls
```

5. Open **Document Outline** (View > Other Windows > Document Outline), select 'ContentPanel', and add a Button again.
6. Observe that it is now correctly added to the internal panel.

### Expected Behaviour

Clicking inside a design-surface-exposed internal panel should select it, and controls dragged in should be added to that panel, **not the root UserControl**.

### Actual Behaviour

- Clicking in the designer surface always selects the UserControl itself.
- Controls are always added to the root `Controls` collection, even when clicking inside the exposed sub-panel.
- Only by selecting the sub-panel in Document Outline can controls be added correctly.

### Why this Matters

- Breaks the designer experience for any composite/custom controls.
- Causes confusion, misplaced controls, and wasted developer time.
- No obvious fix/workaround except using Document Outline, which is not discoverable or intuitive for most developers.

### Environment

- .NET 9 (also occurs in .NET 6/7/8, .NET Framework 4.8.1)
- Visual Studio 2022 v17.10.x

### Workarounds Tried

- Using Document Outline: works but is not intuitive.
- Custom designers/overrides: no effect.
- Runtime logic: not relevant for design-time code.

### Suggested Solution

- Support hit-testing/selecting 'EnableDesignMode' sub-controls by mouse click in the designer.
- At least provide an option or shortcut to route drop/selection to the correct design surface.

---

**Thank you for considering!**

### .NET version

.NET 9

### Did this work in a previous version of Visual Studio and/or previous .NET release?

.NET Framework 4.8.1

### Issue description

When building a custom UserControl that exposes an internal panel with `EnableDesignMode`, it is not possible to select this internal panel by clicking in the designer. All drag-and-drop operations add controls to the root UserControl, not the design-surface-exposed panel, unless the Document Outline is used.

### Steps to reproduce

1. Add the following minimal UserControl and designer to a .NET WinForms project:

```
using System.ComponentModel;
using System.Windows.Forms;
using System.Windows.Forms.Design;

[Designer(typeof(DemoPanelDesigner))]
public class DemoPanel : UserControl
{
private readonly Panel _contentPanel = new Panel();

public DemoPanel()
{
_contentPanel.Dock = DockStyle.Fill;
this.Controls.Add(_contentPanel);
}

[Browsable(true)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
public Panel ContentPanel => _contentPanel;
}

public class DemoPanelDesigner : ParentControlDesigner
{
public override void Initialize(IComponent component)
{
base.Initialize(component);

if (component is DemoPanel panel)
EnableDesignMode(panel.ContentPanel, nameof(DemoPanel.ContentPanel));
}
}
```

2. Build the project, open a Form in the designer, and add `DemoPanel` to it.
3. Try to drag a Button (or any control) and drop it inside the `DemoPanel` surface.
4. Observe the generated code:

### Diagnostics

```text

```