enable updates if there's a visual with render critical timing on it.

samples/ControlCatalog/Pages/TimeCriticalRenderPage.cs

@@ -21,7 +21,7 @@ namespace ControlCatalog.Pages
 
         private Typeface _typeface = Typeface.Default;
 
-        public void ThreadSafeRender(DrawingContext context, Size logicalSize, double scaling)
+        public override void Render(DrawingContext context)
         {
             var nowTs = _st.Elapsed;
             var now = DateTime.Now;
@@ -43,9 +43,14 @@ namespace ControlCatalog.Pages
             };
             var back = new ImmutableSolidColorBrush(Colors.LightGray);
             var textBrush = new ImmutableSolidColorBrush(Colors.Black);
-            context.FillRectangle(back, new Rect(logicalSize));
+            context.FillRectangle(back, new Rect(0,0,12,12));
             context.DrawText(textBrush, new Point(5, 5), fmt);
             _frame++;
         }
+
+        public void ThreadSafeRender(DrawingContext context, Size logicalSize, double scaling)
+        {
+           
+        }
     }
 }

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -250,7 +250,7 @@ namespace Avalonia.Rendering
             }
         }
 
-        bool NeedsUpdate => _dirty == null || _dirty.Count > 0;
+        private bool NeedsUpdate => _dirty == null || _dirty.Count > 0 || (_scene?.Item?.HasCriticalVisual ?? false);
         bool IRenderLoopTask.NeedsUpdate => NeedsUpdate;
 
         void IRenderLoopTask.Update(TimeSpan time) => UpdateScene();
@@ -631,6 +631,7 @@ namespace Avalonia.Rendering
                     foreach (var visual in _recalculateChildren)
                     {
                         var node = scene.FindNode(visual);
+                        
                         ((VisualNode)node)?.SortChildren(scene);
                     }
 

src/Avalonia.Visuals/Rendering/SceneGraph/Scene.cs

@@ -25,12 +25,14 @@ namespace Avalonia.Rendering.SceneGraph
                 new VisualNode(rootVisual, null),
                 new Dictionary<IVisual, IVisualNode>(),
                 new SceneLayers(rootVisual),
-                0)
+                0,
+                false)
         {
             _index.Add(rootVisual, Root);
         }
 
-        private Scene(VisualNode root, Dictionary<IVisual, IVisualNode> index, SceneLayers layers, int generation)
+        private Scene(VisualNode root, Dictionary<IVisual, IVisualNode> index, SceneLayers layers, int generation,
+            bool hasCriticalVisual)
         {
             Contract.Requires<ArgumentNullException>(root != null);
 
@@ -41,6 +43,7 @@ namespace Avalonia.Rendering.SceneGraph
             Layers = layers;
             Generation = generation;
             root.LayerRoot = root.Visual;
+            HasCriticalVisual = hasCriticalVisual;
         }
 
         public Task Rendered => _rendered.Task;
@@ -70,6 +73,9 @@ namespace Avalonia.Rendering.SceneGraph
         /// </summary>
         public double Scaling { get; set; } = 1;
 
+        public bool HasCriticalVisual { get;
+            set; }
+
         /// <summary>
         /// Adds a node to the scene index.
         /// </summary>
@@ -90,7 +96,7 @@ namespace Avalonia.Rendering.SceneGraph
             var index = new Dictionary<IVisual, IVisualNode>(_index.Count);
             var root = Clone((VisualNode)Root, null, index);
 
-            var result = new Scene(root, index, Layers.Clone(), Generation + 1)
+            var result = new Scene(root, index, Layers.Clone(), Generation + 1, HasCriticalVisual)
             {
                 Size = Size,
                 Scaling = Scaling,

src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs

@@ -49,6 +49,11 @@ namespace Avalonia.Rendering.SceneGraph
                 UpdateSize(scene);
             }
 
+            if (visual is IRenderTimeCriticalVisual criticalVisual)
+            {
+                ForceUpdateCriticalRender(scene, visual);
+            }
+
             if (visual.VisualRoot == scene.Root.Visual)
             {
                 if (node?.Parent != null &&
@@ -298,10 +303,19 @@ namespace Avalonia.Rendering.SceneGraph
             }
         }
         
+        // TODO: Im clueless so im just copying the above method to force a dirty rect.
+        private void ForceUpdateCriticalRender(Scene scene, IVisual visual)
+        {
+         }
+        
         private static VisualNode CreateNode(Scene scene, IVisual visual, VisualNode parent)
         {
             var node = new VisualNode(visual, parent);
             node.LayerRoot = parent.LayerRoot;
+            if (visual is IRenderTimeCriticalVisual)
+            {
+                scene.HasCriticalVisual = true;
+            }
             scene.Add(node);
             return node;
         }
@@ -315,6 +329,12 @@ namespace Avalonia.Rendering.SceneGraph
                     Deindex(scene, visual);
                 }
             }
+            
+            if (node.Visual is IRenderTimeCriticalVisual)
+            {
+                scene.HasCriticalVisual = false;
+            }
+            
             scene.Remove(node);
 
             node.SubTreeUpdated = true;