fix(scrollviewer): Add ShouldFallBackToNativeScrollBars flag

When set to False on ScrollViewer as an attached property, this will prevent native scrollbars showing when no ScrollBar is found in the ControlTemplate. Defaults to true, for compatibility with older versions of the default style and platforms where ScrollBar is not yet fully supported.

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

@@ -58,5 +58,32 @@ public static void SetUpdatesMode(Windows.UI.Xaml.Controls.ScrollViewer scrollVi
 		/// <returns>The updates mode of the <paramref name="scrollViewer"/>.</returns>
 		public static ScrollViewerUpdatesMode GetUpdatesMode(Windows.UI.Xaml.Controls.ScrollViewer scrollViewer)
 			=> (ScrollViewerUpdatesMode)scrollViewer.GetValue(UpdatesModeProperty);
+
+		/// <summary>
+		/// Getter for ShouldFallBackToNativeScrollBars attached property. If true, and no <see cref="Windows.UI.Xaml.Primitives.ScrollBar"/> is
+		/// found in <paramref name="scrollViewer"/>'s template, then native scroll bars (for platforms where available) will be shown instead.
+		/// If false, no scroll bars will be shown. True by default, for backward-compatibility.
+		/// </summary>
+		/// <param name="scrollViewer"></param>
+		/// <returns></returns>
+		public static bool GetShouldFallBackToNativeScrollBars(Windows.UI.Xaml.Controls.ScrollViewer scrollViewer)
+		{
+			return (bool)scrollViewer.GetValue(ShouldFallBackToNativeScrollBarsProperty);
+		}
+
+		/// <summary>
+		/// Setter for ShouldFallBackToNativeScrollBars attached property. If true, and no <see cref="Windows.UI.Xaml.Primitives.ScrollBar"/> is
+		/// found in <paramref name="scrollViewer"/>'s template, then native scroll bars (for platforms where available) will be shown instead.
+		/// If false, no scroll bars will be shown. True by default, for backward-compatibility.
+		/// </summary>
+		public static void SetShouldFallBackToNativeScrollBars(Windows.UI.Xaml.Controls.ScrollViewer scrollViewer, bool value)
+		{
+			scrollViewer.SetValue(ShouldFallBackToNativeScrollBarsProperty, value);
+		}
+
+		public static readonly DependencyProperty ShouldFallBackToNativeScrollBarsProperty =
+			DependencyProperty.RegisterAttached("ShouldFallBackToNativeScrollBars", typeof(bool), typeof(ScrollViewer), new PropertyMetadata(true));
+
+
 	}
 }

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

@@ -774,11 +774,11 @@ private static bool ComputeIsScrollEnabled(double scrollable, ScrollBarVisibilit
 				&& visibility != ScrollBarVisibility.Disabled
 				&& mode != ScrollMode.Disabled;
 
-		private static ScrollBarVisibility ComputeNativeScrollBarVisibility(ScrollBarVisibility visibility, ScrollMode mode, ScrollBar? managedScrollbar)
+		private ScrollBarVisibility ComputeNativeScrollBarVisibility(ScrollBarVisibility visibility, ScrollMode mode, ScrollBar? managedScrollbar)
 			=> mode switch
 			{
 				ScrollMode.Disabled => ScrollBarVisibility.Disabled,
-				_ when managedScrollbar is null => visibility,
+				_ when managedScrollbar is null && Uno.UI.Xaml.Controls.ScrollViewer.GetShouldFallBackToNativeScrollBars(this) => visibility,
 				_ when visibility == ScrollBarVisibility.Disabled => ScrollBarVisibility.Disabled,
 				_ => ScrollBarVisibility.Hidden // If a managed scroll bar was set in the template, native scroll bar has to stay Hidden
 			};