Add WrapPanel API Spec based on Windows Community Toolkit (WCT)

specs/WCT/Primitives/WrapPanel.md

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+
+WrapPanel
+===
+
+# Background
+
+_This spec brings WrapPanel from the Windows Community Toolkit (as WPF Polyfill) to WinUI._
+
+WrapPanel has been a staple of XAML development since its inception, originally included in WPF 
+[WrapPanel](https://learn.microsoft.com/dotnet/desktop/wpf/controls/wrappanel). It was originally added 
+to the Windows Community Toolkit (WCT) in v1.3 on 2/10/2017 
+([see WCT history here](https://github.com/CommunityToolkit/Windows/discussions/722)) and used in a 
+multitude of applications since.
+
+`WrapPanel` is a simple and versatile panel that positions child controls vertically or horizontally 
+based on the orientation in sequence and when max width / max height is reached a new virtual row (in 
+case of horizontal) or column (in case of vertical) is automatically created to fit new controls.
+
+This differs importantly from `StackPanel` where all children must be in a single row/column and aids in 
+responsive UI flow.
+
+> [!IMPORTANT]
+> About WPF Compatibility: This spec aligns to the WCT implementation for compatibility for existing 
+WinUI users. WrapPanel in the WCT was modernized and updated as part of general Panel modernization for 
+responsive layouts done for UWP and simplified for developer's ease-of-use. It is the goal to bring this 
+to WinUI for this same fundamental experience and allow easy migration from WCT developers and existing 
+usage in WinUI apps vs. direct migration for WPF developers.
+
+Reference Docs:
+- [WCT WrapPanel Conceptual](https://learn.microsoft.com/dotnet/communitytoolkit/windows/primitives/wrappanel)
+- [WCT WrapPanel API](https://learn.microsoft.com/dotnet/api/communitytoolkit.winui.controls.wrappanel)
+- [WinUI StackPanel Conceptual](https://learn.microsoft.com/windows/apps/design/layout/layout-panels#stackpanel)
+- [WinUI StackPanel API Comparison](https://learn.microsoft.com/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.controls.stackpanel)
+- [WPF WrapPanel Conceptual](https://learn.microsoft.com/dotnet/desktop/wpf/controls/wrappanel)
+- [WPF WrapPanel API](https://learn.microsoft.com/dotnet/api/system.windows.controls.wrappanel)
+- [WPF StackPanel API Comparison](https://learn.microsoft.com/dotnet/api/system.windows.controls.stackpanel)
+
+
+# Conceptual pages (How To)
+
+<!-- _(This is conceptual documentation that will go to docs.microsoft.com "how to" page)_ -->
+
+<!-- See: https://learn.microsoft.com/windows/apps/design/layout/layout-panels -->
+
+The WrapPanel is a layout panel that arranges child elements in a sequential position from left to right, 
+items overflowing the line will break to the next line automatically at the edge of the containing panel. 
+
+You can set the `Orientation` property to specify the direction of child elements. The default 
+orientation is **Horizontal**.
+
+The following XAML shows how to create a WrapPanel of items:
+
+```xml
+    <WrapPanel Width="132">
+        <Rectangle Fill="Red" Width="44" Height="44"/>
+        <Rectangle Fill="Blue" Width="44" Height="44"/>
+        <Rectangle Fill="Green" Width="44" Height="44"/>
+        <Rectangle Fill="Orange" Width="44" Height="44"/>
+    </WrapPanel>
+```
+
+The result looks like this:
+
+![WrapPanel example layout](images/layout-panel-wrap-panel.png)
+
+In a WrapPanel, if a child element's size is not set explicitly, it will be given the minimal amount 
+of space for layout. In this example, the size of the rectangles are explicitly set, if the 
+**Width** or **Height** were not provided, the rectangle would not appear (as it would be given a 
+**0** size in either dimension). This is a key contrast to a standard `StackPanel` control in terms 
+of default behavior.
+
+# API Pages
+
+<!-- _(Each of the following L2 sections correspond to a page that will be on docs.microsoft.com)_ -->
+
+## WrapPanel class
+
+The WrapPanel is a layout panel that arranges child elements in a sequential position from left to right, 
+items overflowing the line will break to the next line automatically at the edge of the containing panel. 
+
+![WrapPanel example layout](images/WrapPanel.png)
+
+By default, WrapPanel stacks items from left to right in the order they are declared. You can set the 
+`Orientation` property to **Vertical** to stack items from top to bottom instead, overflowing to the 
+right.
+
+Spacing can be automatically added between items using the `HorizontalSpacing` and `VerticalSpacing` 
+properties. When the `Orientation` is **Horizontal**, `HorizontalSpacing` adds uniform horizontal spacing 
+between each individual item, and `VerticalSpacing` adds uniform spacing between each row of items.
+
+When the `Orientation` is **Vertical**, `HorizontalSpacing` adds uniform spacing between each column of 
+items, and `VerticalSpacing` adds uniform vertical spacing between individual items.
+
+## WrapPanel.HorizontalSpacing property
+
+Gets or sets a uniform Horizontal distance (in pixels) between items when Orientation is set to 
+Horizontal, or between columns of items when Orientation is set to Vertical.
+
+In the following example, HorizontalSpacing has been added to space out items, though in this case that 
+then wraps the Green box around to the next line due to the Width constraint on the WrapPanel:
+
+```xml
+    <controls:WrapPanel Width="132" HorizontalSpacing="16">
+        <Rectangle Fill="Red" Width="44" Height="44"/>
+        <Rectangle Fill="Blue" Width="44" Height="44"/>
+        <Rectangle Fill="Green" Width="44" Height="44"/>
+        <Rectangle Fill="Orange" Width="44" Height="44"/>
+    </controls:WrapPanel>
+```
+
+The result looks like this:
+
+![WrapPanel HorizontalSpacing example](images/WrapPanelHorizontalSpacing.png)
+
+## WrapPanel.Orientation property
+
+Gets or sets the orientation of the WrapPanel. **Horizontal** means that child controls will be added 
+horizontally until the width of the panel is reached, then a new row is added to add new child controls.
+**Vertical** means that children will be added vertically until the height of the panel is reached, then 
+a new column is added. Default is **Horizontal**.
+
+## WrapPanel.Padding property
+
+Gets or sets the distance between the border and its child object.
+
+## WrapPanel.StretchChild property
+
+Gets or sets how the last child item in the WrapPanel is laid out. When set to **Last**, the final child 
+can stretch and be given all the rest of the available space for the row (when Orientation is 
+**Horizontal**) or column (when Orientation is **Vertical**). Defaults to **None** where no changes to 
+default behavior layout are provided.
+
+In the following example, the last Orange rectangle will be constrained and pushed to the 2nd row. 
+Normally if given the same Width/Height as the others, it'd sit just below the Red rectangle to the left. 
+Instead, we can remove it's Width and have it fill the rest of the row by setting `StretchChild` to **Last**:
+
+```xml
+    <controls:WrapPanel Width="132" StretchChild="Last">
+        <Rectangle Fill="Red" Width="44" Height="44"/>
+        <Rectangle Fill="Blue" Width="44" Height="44"/>
+        <Rectangle Fill="Green" Width="44" Height="44"/>
+        <Rectangle Fill="Orange" Height="44" />
+    </controls:WrapPanel>
+```
+
+The result looks like this:
+
+![WrapPanel StretchChild example](images/WrapPanelStretchChild.png)
+
+## Other MyExample members
+
+| Name | Description |
+|-|-|
+| VerticalSpacing | Gets or sets a uniform Vertical distance (in pixels) between items when Orientation is set to Vertical, or between rows of items when Orientation is set to Horizontal. (defaults to 0) |
+
+# API Details
+
+```c# (but really MIDL3)
+namespace Microsoft.UI.Xaml.Controls
+{
+  runtimeclass WrapPanel
+  {
+      double HorizontalSpacing { get; set; }
+      Orientation Orientation { get; set; }
+      Thickness Padding { get; set; }
+      StretchChild StretchChild { get; set; }
+      double VerticalSpacing { get; set; }      
+  }
+
+  enum StretchChild
+  {
+    None = 0,
+    Last
+  }
+}
+```
+
+# Appendix
+
+<!-- TEMPLATE
+  Anything else that you want to write down about implementation notes and for posterity,
+  but that isn't necessary to understand the purpose and usage of the API.
+
+  This or the Background section are a good place to describe alternative designs
+  and why they were rejected.
+-->