DisplayArea.GetFromDisplayId (#2868)

* Creating DisplayArea.md

* Resolving PR comments

specs/Windowing/DisplayArea.md

@@ -0,0 +1,75 @@
+DisplayArea API update to facilitate easier use for developers working with DisplayIds
+===
+
+# Background
+Up until WinAppSDK 1.1, the only easy mapping between a
+[Microsoft.UI.DisplayId](https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.displayid?view=windows-app-sdk-1.1_)
+and a
+[Microsoft.UI.Windowing.DisplayArea](https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.windowing.displayarea?view=windows-app-sdk-1.1_)
+was if the developer started with a DisplayArea and wanted to find the DisplayId it maps to. If a
+developer wanted to find out what DisplayArea is configured for a given DisplayId they would have
+to enumerate **all** DisplayAreas using the static method
+[DisplayArea.FindAll()](https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.windowing.displayarea.findall?view=windows-app-sdk-1.1)
+and then compare each DisplayArea returned with the DisplayId for the display they were interested in.
+
+This feature is remedying that complexity by introducing another static method on the DisplayArea
+class. `DisplayArea.GetFromDisplayId(DisplayId)` returns a DisplayArea that corresponds to the
+specified DisplayId, if one exists. If more than one such DisplayArea exists, the system chooses
+one to return. This choice will remain consistent until the next display topology change.
+
+This matches the pattern we already have in place for the following apis:
+1. DisplayArea given a WindowId -
+ [DisplayArea.GetFromWindowId(WindowId, DisplayAreaFallBack)](https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.windowing.displayarea.getfromwindowid?view=windows-app-sdk-1.1)
+
+2. AppWindow given a WindowId -
+[AppWindow.GetFromWindowId(WindowId)](https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.windowing.appwindow.getfromwindowid?view=windows-app-sdk-1.1)
+
+# API Pages
+
+## DisplayArea.GetFromDisplayId static method
+
+This is the new static method added to WinAppSdk 1.2.
+
+The following example shows how an AppWindow could be configured to fully occupy the DisplayArea
+associated with the passed hmonitor/displayId:
+
+```csharp
+using Microsoft.UI;
+using Microsoft.UI.Windowing;
+using Windows.Graphics;
+
+void ExampleUsage_GetDisplayAreaForDisplay(AppWindow appWindow, IntPtr hmonitor)
+{
+    DisplayId displayId = Win32Interop.GetDisplayIdFromMonitor(hmonitor);
+    DisplayArea displayArea = DisplayArea.GetFromDisplayId(displayId);
+
+    if(displayArea != null)
+    {
+        RectInt32 rect = new RectInt32()
+        {
+            X = 0,
+            Y = 0,
+            Width = displayArea.WorkArea.Width,
+            Height = displayArea.WorkArea.Height
+        };
+        appWindow.MoveAndResize(rect, displayArea);
+    }
+}
+```
+
+# API Details
+
+```csharp
+namespace Microsoft.UI.Windowing
+{
+    [contract(Microsoft.Foundation.WindowsAppSDKContract, 1)]
+    runtimeclass DisplayArea
+    {
+        [contract(Microsoft.Foundation.WindowsAppSDKContract, 1.2)]
+        {
+            // Queries what DisplayArea is configured for the display identified by displayId.
+            static DisplayArea GetFromDisplayId(Microsoft.UI.DisplayId displayId);
+        }
+    }
+}
+```