fix: Popup should receive focus if its child is focusable

Author: MartinZikmund

src/Uno.UI/UI/Xaml/Controls/Popup/Popup.cs

@@ -25,6 +25,12 @@ public partial class Popup : PopupBase
 
 		internal bool IsSubMenu { get; set; }
 
+		/// <summary>
+		/// In WinUI, Popup has IsTabStop set to true by default.
+		/// UWP does not include IsTabStop, but Popup is still focusable.
+		/// </summary>
+		private protected override bool IsTabStopDefaultValue => true;
+
 		/// <summary>
 		/// Returns true if the popup should show the light-dismiss overlay with its current configuration, false if not
 		/// </summary>

src/Uno.UI/UI/Xaml/Controls/Popup/PopupBase.cs

@@ -78,18 +78,27 @@ partial void OnIsOpenChangedPartial(bool oldIsOpen, bool newIsOpen)
 						_lastFocusedElement = new WeakReference<UIElement>(focusedElement);
 						_lastFocusState = focusState;
 					}
+
+					// Give the child focus if allowed
+					if (Child is FrameworkElement fw && fw.AllowFocusOnInteraction)
+					{
+						Focus(FocusState.Programmatic);
+					}
 				}
 
 				Opened?.Invoke(this, newIsOpen);
 			}
 			else
 			{
 				_openPopupRegistration?.Dispose();
-				
-				if (_lastFocusedElement != null && _lastFocusedElement.TryGetTarget(out var target))
+
+				if (IsLightDismissEnabled)
 				{
-					target.Focus(_lastFocusState);
-					_lastFocusedElement = null;
+					if (_lastFocusedElement != null && _lastFocusedElement.TryGetTarget(out var target))
+					{
+						target.Focus(_lastFocusState);
+						_lastFocusedElement = null;
+					}
 				}
 
 				Closed?.Invoke(this, newIsOpen);