Fix XAML popup positioning and light dismiss in ScrollView (#15557)

[Nitin-100]

Description

This PR fixes Issue #15557 - "Pop-ups of Xaml controls need positioning and dismissal"

When XAML controls with popups (like ComboBox, DatePicker, TimePicker) are hosted inside a React Native ScrollView via ContentIslandComponentView, two bugs occur:

1. Bug 1 (Position): After scrolling, opening the popup shows it at the wrong position (offset by the scroll amount)
2. Bug 2 (Light Dismiss): Opening a popup and then scrolling leaves the popup floating at its original position instead of dismissing

Root Cause

• Bug 1: ContentIslandComponentView uses ChildSiteLink.LocalToParentTransformMatrix for popup positioning, but wasn't being notified when scroll position changed
• Bug 2: XAML light dismiss behavior wasn't being triggered when scroll begins in React Native

Solution

Bug 1 Fix: Popup Position After Scroll

• ScrollViewComponentView now fires LayoutMetricsChanged event when scroll position changes
• ContentIslandComponentView listens to this and calls ParentLayoutChanged()
• This updates LocalToParentTransformMatrix with the correct scroll offset via getClientRect()

Bug 2 Fix: Light Dismiss on Scroll (Generic for ALL 3rd Party XAML Controls)

• Added SetXamlRoot(XamlRoot) method to ContentIslandComponentView API
• 3rd party XAML components register their XamlRoot after the element is loaded
• When scroll begins, DismissPopups() uses VisualTreeHelper.GetOpenPopupsForXamlRoot() to find and close ALL open XAML popups
• This is a generic solution that works for ANY XAML control (ComboBox, DatePicker, TimePicker, Flyouts, etc.)

Files Changed

Core Fix (vnext/Microsoft.ReactNative/)

• CompositionComponentView.idl - Added SetXamlRoot() method to ContentIslandComponentView
• Fabric/Composition/ContentIslandComponentView.h/.cpp - Implemented SetXamlRoot() and DismissPopups()
• Fabric/Composition/ScrollViewComponentView.h/.cpp - Added scroll position change notification and popup dismissal trigger

Sample Component (packages/sample-custom-component/)

• Added ComboBox component as example of proper 3rd party implementation
• Shows how to use SetXamlRoot() for popup dismissal

Test Samples

• XamlPopupRepro.windows.tsx - Test case in RNTester
• xamlPopupBug.tsx - Test case in Playground

---

🔧 Guidance for 3rd Party XAML Component Developers

If you're building a custom XAML component that has popups (ComboBox, DatePicker, Flyouts, etc.), follow this pattern to ensure proper popup behavior inside React Native ScrollViews:

C++ Implementation Pattern

void YourComponentView::InitializeContentIsland(
    const winrt::Microsoft::ReactNative::Composition::ContentIslandComponentView &islandView) noexcept {
  
  // 1. Create your XamlIsland and XAML control
  m_xamlIsland = winrt::Microsoft::UI::Xaml::XamlIsland{};
  m_yourControl = winrt::Microsoft::UI::Xaml::Controls::YourControl{};
  
  // 2. Set up the control
  // ... configure your control ...
  
  // 3. Set content and connect
  m_xamlIsland.Content(m_yourControl);
  islandView.Connect(m_xamlIsland.ContentIsland());
  
  // 4. IMPORTANT: Register XamlRoot for popup dismissal (Issue #15557)
  // This enables automatic popup closing when parent ScrollView scrolls
  m_yourControl.Loaded([islandView, this](auto const &, auto const &) {
    if (auto xamlRoot = m_yourControl.XamlRoot()) {
      islandView.SetXamlRoot(xamlRoot);
    }
  });
}

Why This Works

1. Position Fix: The framework automatically updates LocalToParentTransformMatrix when scroll position changes, so your popup appears at the correct location.

2. Light Dismiss: When you register your XamlRoot, the framework uses VisualTreeHelper.GetOpenPopupsForXamlRoot() to find ALL open popups when scroll begins and closes them automatically.

What else handled with this fix

• Correct popup positioning after scroll
• Automatic popup dismissal when scroll starts (light dismiss)
• Works with ANY XAML popup type (ComboBox dropdown, DatePicker calendar, Flyouts, etc.)
• No component-specific code needed in the framework

---

Testing

1. Run the XamlPopupRepro sample in Playground
2. Scroll down in the ScrollView
3. Click a ComboBox to open dropdown - should appear at correct position
4. Open a ComboBox dropdown, then scroll - dropdown should close automatically

Microsoft Reviewers: Open in CodeFlow

[sundaramramaswamy]

Please address comments.

[sundaramramaswamy]

ContentIsland is generic. Xaml 3P modules just ride on it but any other island (inherited from Microsoft.Ui.ContentIsland) can be hosted. The core "knowing" about Xaml is a problem. Instead of the core dismissing the popup (and there by knowing about Xaml) it should just tell the module to dismiss pop-up if any.

Try to see if you could emit an existing event to which the 3P component can listen and dismiss pop-ups.

[sundaramramaswamy]

Please drop the re-formatting and only keep your change. Two reasons

1. What changed isn't evident
2. It screws up git blame output