fix(menuflyout): Ensure MenuFlyout nested submenus stay on screen

Fix bug where nested submenus could be shown offscreen on the 2nd and later times they appeared.

src/Uno.UI/UI/Xaml/Controls/MenuFlyout/CascadingMenuHelper.cs

@@ -53,6 +53,9 @@ internal class CascadingMenuHelper
 		// This fallback is used if we fail to retrieve a value from the MenuShowDelay RegKey
 		const int DefaultMenuShowDelay = 400; // in milliseconds
 
+		// Uno-specific workaround (see comment below)
+		private Point? _lastTargetPoint;
+
 		public CascadingMenuHelper()
 		{
 			m_isPointerOver = false;
@@ -567,6 +570,13 @@ internal void OpenSubMenu()
 						}
 
 						ownerAsSubMenuOwner.OpenSubMenu(targetPoint);
+						if (_lastTargetPoint is { } lastTargetPoint)
+						{
+							// Uno-specific workaround: reapply the location calculated in OnPresenterSizeChanged(), since that one properly
+							// adjusts to keep submenu within screen bounds. (WinUI seemingly relies upon presenter.SizeChanged being raised
+							// every time submenu opens? On Uno it isn't.)
+							ownerAsSubMenuOwner.PositionSubMenu(lastTargetPoint);
+						}
 						ownerAsSubMenuOwner.RaiseAutomationPeerExpandCollapse(true /* isOpen */);
 						ElementSoundPlayer.RequestInteractionSoundForElement(ElementSoundKind.Invoke, ownerAsControl);
 					}
@@ -932,6 +942,7 @@ public void OnVisibilityChanged()
 					}
 				}
 
+				_lastTargetPoint = targetPoint;
 				ownerAsSubMenuOwner.PositionSubMenu(targetPoint);
 			}
 		}