fix(perf): keep PerfStress responsive at 250/500 elements

Two complementary fixes for the Reactor.TestApp PerfStress demo, where the
UI froze under sustained high-frequency setState from an async sort loop.

ReactorHost / ReactorHostControl: RequestRender already coalesces concurrent
setStates within one dispatch tick, but only demoted to LOW priority when a
state change happened *during* a render. The PerfStress pattern (setState
between `await Task.Delay` ticks) never hit that path, so renders piled up at
Normal priority and starved input/layout/paint on the same UI thread.
RenderPriorityPolicy now demotes the next TryEnqueue to Low whenever the
previous render exceeded a 16 ms frame budget — same mechanism the
existing in-render path already uses, just extended to the
between-renders path.

PerfStressDemo: each render allocated ~1500 fresh SolidColorBrush instances
at 500 elements (3 per bar × string `.Background("#hex")` overload), and
because every brush had a different reference, UpdateBorder wrote
b.Background unconditionally and WinUI re-painted every border every tick.
A [ThreadStatic] brush cache keyed by color string collapses that to ~16
brushes for the lifetime of the demo and lets the reconciler's reference
check short-circuit the redundant Background writes.

Tests:
- 9 new RenderPriorityPolicyTests pin the priority decisions and the
  ReactorHost/ReactorHostControl wiring contract.
- Full unit suite (7534 tests) and selftest suite (689 tests) pass.

Author: codemonkeychris

samples/Reactor.TestApp/Demos/PerfStressDemo.cs

@@ -9,6 +9,7 @@
 using Microsoft.UI.Reactor.Controls;
 using static Microsoft.UI.Reactor.Factories;
 using static Microsoft.UI.Reactor.Core.Theme;
+using WinMedia = Microsoft.UI.Xaml.Media;
 
 class PerfStressDemo : Component
 {
@@ -20,6 +21,32 @@ record SortState(int[] Values, int[] Colors, int Pivot, int Left, int Right, boo
         "#4dd0e1", "#aed581", "#ffd54f", "#e57373", "#9575cd",
     ];
 
+    // Brush cache: at 500 bars × 3 colored sub-elements per bar, the string
+    // overload of .Background() would allocate ~1500 SolidColorBrush instances
+    // per render tick — and because each new brush is reference-different,
+    // UpdateBorder writes Background unconditionally and WinUI re-paints every
+    // border every tick. Caching one brush per color string per host thread
+    // collapses that to ~1500 dictionary lookups per render and lets the
+    // reconciler's reference-equality check on Background short-circuit.
+    //
+    // SolidColorBrush is a DependencyObject — it has thread affinity, so the
+    // cache is keyed by managed thread id. PerfStress only renders on one UI
+    // thread; the per-thread keying is defensive for hot reload / multi-window
+    // futures.
+    [ThreadStatic]
+    private static Dictionary<string, WinMedia.SolidColorBrush>? t_brushCache;
+
+    static WinMedia.SolidColorBrush Brush(string color)
+    {
+        var cache = t_brushCache ??= new Dictionary<string, WinMedia.SolidColorBrush>(StringComparer.OrdinalIgnoreCase);
+        if (!cache.TryGetValue(color, out var brush))
+        {
+            brush = BrushHelper.Parse(color);
+            cache[color] = brush;
+        }
+        return brush;
+    }
+
     public override Element Render()
     {
         var (elementCount, setElementCount) = UseState(100);
@@ -233,11 +260,14 @@ async Task QSort(int lo, int hi)
                     int val = sortState.Values[i];
 
                     // Each bar contains child controls to stress the reconciler:
-                    // a tiny progress indicator + a value label + a colored pip
+                    // a tiny progress indicator + a value label + a colored pip.
+                    // All Background() calls go through the cached brush so the
+                    // reconciler sees stable brush references between renders
+                    // and skips redundant WinUI Background writes.
                     Element barContent = VStack(0,
                         // Top: small colored indicator pip (changes with sort state)
                         Border(Empty())
-                            .Background(isPivot ? "#ffffff" : isActive ? "#ffeb3b" : BarColors[(colorIdx + 1) % BarColors.Length])
+                            .Background(Brush(isPivot ? "#ffffff" : isActive ? "#ffeb3b" : BarColors[(colorIdx + 1) % BarColors.Length]))
                             .CornerRadius(1)
                             .Width(Math.Min(barWidth - 1, 6))
                             .Height(2),
@@ -247,15 +277,15 @@ async Task QSort(int lo, int hi)
                             : Empty(),
                         // Bottom: progress-like fill showing relative position
                         Border(Empty())
-                            .Background(BarColors[(colorIdx + 2) % BarColors.Length])
+                            .Background(Brush(BarColors[(colorIdx + 2) % BarColors.Length]))
                             .CornerRadius(0)
                             .Width(Math.Max(1, barWidth * 0.6))
                             .Height(Math.Max(1, barHeight * 0.15))
                             .Opacity(0.5)
                     );
 
                     Element bar = Border(barContent)
-                        .Background(BarColors[colorIdx])
+                        .Background(Brush(BarColors[colorIdx]))
                         .CornerRadius(0)
                         .Width(barWidth)
                         .Height(barHeight)
@@ -270,7 +300,7 @@ async Task QSort(int lo, int hi)
                 return HStack(0, barElements).Height(220).VAlign(VerticalAlignment.Bottom);
             }))
                 .CornerRadius(8)
-                .Background("#1a1a2e")
+                .Background(Brush("#1a1a2e"))
                 .Padding(8),
 
             // Performance stats
@@ -315,7 +345,7 @@ async Task QSort(int lo, int hi)
                         double h = Math.Min(50, t * 10); // 1ms = 10px
                         string color = t < 2 ? "#81c784" : t < 8 ? "#fff176" : t < 16 ? "#ff8a65" : "#e57373";
                         return (Element)Border(Empty())
-                            .Background(color)
+                            .Background(Brush(color))
                             .CornerRadius(0)
                             .Width(Math.Max(1, 600.0 / 100))
                             .Height(Math.Max(1, h))
@@ -337,7 +367,7 @@ async Task QSort(int lo, int hi)
 
     static Element LegendItem(string color, string label) =>
         HStack(4,
-            Border(Empty()).Background(color).CornerRadius(2).Width(12).Height(12),
+            Border(Empty()).Background(Brush(color)).CornerRadius(2).Width(12).Height(12),
             Caption(label).Foreground(SecondaryText)
         );
 }

src/Reactor/Hosting/ReactorHost.cs

@@ -104,6 +104,12 @@ public sealed class ReactorHost : IDisposable
     private readonly Stopwatch _reportClock = Stopwatch.StartNew();
     private long _totalRenderCount;
 
+    // Last render's total duration (tree + reconcile + effects), in ms.
+    // Read by RequestRender to demote the next enqueue to Low priority when a
+    // slow render is starving the dispatcher of input/layout/paint slots.
+    // See RenderPriorityPolicy.
+    private double _lastRenderMs;
+
     // Public perf snapshot — updated every ~1 second, readable from components
     private RenderStats _stats;
 
@@ -480,7 +486,14 @@ internal void RequestRender(bool force = false)
             return;
         }
 
-        _dispatcherQueue.TryEnqueue(RenderLoop);
+        // Demote to Low priority when the previous render exceeded the frame
+        // budget — high-frequency setState sources (animation, simulation,
+        // streaming data) otherwise pack the dispatcher with back-to-back
+        // Normal-priority renders and starve input/layout/paint. See
+        // RenderPriorityPolicy.
+        _dispatcherQueue.TryEnqueue(
+            RenderPriorityPolicy.PickPriority(_lastRenderMs),
+            RenderLoop);
     }
 
     private void RenderLoop()
@@ -717,6 +730,12 @@ void RecoverFromHookOrder(HookOrderException ex, RenderContext ctx, string mode)
 
             double effectsMs = _phaseSw.Elapsed.TotalMilliseconds;
 
+            // Feed RenderPriorityPolicy so the next RequestRender knows whether
+            // to demote to Low priority. Stored as the most-recent measurement
+            // — no smoothing — so a single slow render is enough to back off,
+            // and a single fast render is enough to return to Normal priority.
+            _lastRenderMs = treeBuildMs + reconcileMs + effectsMs;
+
             OnRenderComplete?.Invoke(treeBuildMs, reconcileMs, effectsMs);
 
 #if DEBUG

src/Reactor/Hosting/ReactorHostControl.cs

@@ -89,6 +89,11 @@ public sealed partial class ReactorHostControl : ContentControl, IDisposable
     private readonly Stopwatch _reportClock = Stopwatch.StartNew();
     private long _totalRenderCount;
 
+    // Last render's total duration (tree + reconcile + effects), in ms.
+    // Read by RequestRender to demote the next enqueue to Low priority when a
+    // slow render is starving the dispatcher of input/layout/paint slots.
+    private double _lastRenderMs;
+
     // Public perf snapshot — updated every ~1 second, readable from components
     private RenderStats _stats;
 
@@ -299,7 +304,12 @@ private void RequestRender()
         // Between renders: CAS 0→1 gates a single TryEnqueue.
         if (Interlocked.CompareExchange(ref _renderPending, 1, 0) != 0) return;
 
-        _dispatcherQueue.TryEnqueue(RenderLoop);
+        // Demote to Low priority after a slow render so input/layout/paint
+        // catch up. See RenderPriorityPolicy and the matching code in
+        // ReactorHost.RequestRender.
+        _dispatcherQueue.TryEnqueue(
+            RenderPriorityPolicy.PickPriority(_lastRenderMs),
+            RenderLoop);
     }
 
     private void RenderLoop()
@@ -509,6 +519,9 @@ void RecoverFromHookOrder(HookOrderException ex, RenderContext ctx, string mode)
 
             double effectsMs = _phaseSw.Elapsed.TotalMilliseconds;
 
+            // Feed RenderPriorityPolicy. See matching note in ReactorHost.Render.
+            _lastRenderMs = treeBuildMs + reconcileMs + effectsMs;
+
             OnRenderComplete?.Invoke(treeBuildMs, reconcileMs, effectsMs);
 
 #if DEBUG

src/Reactor/Hosting/RenderPriorityPolicy.cs

@@ -0,0 +1,41 @@
+using Microsoft.UI.Dispatching;
+
+namespace Microsoft.UI.Reactor.Hosting;
+
+/// <summary>
+/// Decides the dispatcher-queue priority for the next render enqueue based on
+/// recent render duration.
+/// <para>
+/// When a render is faster than a 60 Hz frame, the next render is enqueued at
+/// <see cref="DispatcherQueuePriority.Normal"/> — the default, lowest latency.
+/// When the previous render exceeded the budget, subsequent renders are demoted
+/// to <see cref="DispatcherQueuePriority.Low"/> so that input, layout, and paint
+/// messages on the same UI thread are interleaved between renders. Without this,
+/// a high-frequency state-change source (animation, simulation, streaming data)
+/// can fill the dispatcher with back-to-back renders that starve pointer/keyboard
+/// input — the app feels frozen even though renders are still committing pixels.
+/// </para>
+/// </summary>
+internal static class RenderPriorityPolicy
+{
+    /// <summary>
+    /// Render-duration ceiling beyond which subsequent renders are demoted to
+    /// Low priority. 16 ms is one 60 Hz frame; a render past this point gives
+    /// up its Normal-priority slot so input/layout/paint catch up.
+    /// </summary>
+    public const double DefaultFrameBudgetMs = 16.0;
+
+    /// <summary>
+    /// Decide the priority for the next render enqueue.
+    /// Returns <see cref="DispatcherQueuePriority.Low"/> when the last render
+    /// exceeded the budget, <see cref="DispatcherQueuePriority.Normal"/>
+    /// otherwise (including the cold-start case where no render has run yet
+    /// and <paramref name="lastRenderMs"/> is 0).
+    /// </summary>
+    public static DispatcherQueuePriority PickPriority(
+        double lastRenderMs,
+        double budgetMs = DefaultFrameBudgetMs)
+        => lastRenderMs > budgetMs
+            ? DispatcherQueuePriority.Low
+            : DispatcherQueuePriority.Normal;
+}

tests/Reactor.Tests/Hosting/RenderPriorityPolicyTests.cs

@@ -0,0 +1,119 @@
+using Microsoft.UI.Dispatching;
+using Microsoft.UI.Reactor.Hosting;
+using Xunit;
+
+namespace Microsoft.UI.Reactor.Tests.Hosting;
+
+/// <summary>
+/// The render-loop responsiveness fix for PerfStress at 250/500 elements:
+/// when a render exceeds the 60 Hz frame budget, the host enqueues the next
+/// render at <see cref="DispatcherQueuePriority.Low"/> so the message pump can
+/// interleave input/layout/paint. Without this, back-to-back Normal-priority
+/// renders triggered from <c>await Task.Delay</c> continuations starve UI
+/// input and the app feels frozen until the work finishes.
+///
+/// These tests pin the policy's decisions independent of the dispatcher so
+/// regressions show up at unit-test time rather than as a UI freeze in the
+/// PerfStress demo.
+/// </summary>
+public class RenderPriorityPolicyTests
+{
+    [Fact]
+    public void ColdStart_UsesNormalPriority()
+    {
+        // Before any render has run, lastRenderMs == 0. The host MUST use
+        // Normal priority — Low-priority cold-start would queue behind every
+        // pending dispatcher item and delay first paint noticeably.
+        Assert.Equal(
+            DispatcherQueuePriority.Normal,
+            RenderPriorityPolicy.PickPriority(lastRenderMs: 0));
+    }
+
+    [Fact]
+    public void FastRender_StaysAtNormalPriority()
+    {
+        // A render that fits inside one 60 Hz frame keeps Normal priority.
+        // Demoting fast renders would add latency for no benefit.
+        Assert.Equal(
+            DispatcherQueuePriority.Normal,
+            RenderPriorityPolicy.PickPriority(lastRenderMs: 8));
+    }
+
+    [Fact]
+    public void RenderAtFrameBoundary_StaysAtNormalPriority()
+    {
+        // Exactly at the budget is treated as "fit" — only renders strictly
+        // longer than the budget demote. This avoids flip-flopping when a
+        // render lands right at the boundary.
+        Assert.Equal(
+            DispatcherQueuePriority.Normal,
+            RenderPriorityPolicy.PickPriority(lastRenderMs: RenderPriorityPolicy.DefaultFrameBudgetMs));
+    }
+
+    [Fact]
+    public void SlowRender_DemotesToLowPriority()
+    {
+        // This is the PerfStress fix: a render that exceeds one frame budget
+        // moves the next enqueue to Low priority. The PerfStress scenario
+        // tops out at ~100 ms/render at 500 elements — well past the budget.
+        Assert.Equal(
+            DispatcherQueuePriority.Low,
+            RenderPriorityPolicy.PickPriority(lastRenderMs: 100));
+    }
+
+    [Fact]
+    public void JustOverBudget_DemotesToLowPriority()
+    {
+        // Even a small overrun (just past the 16 ms ceiling) demotes —
+        // the goal is to keep the dispatcher from monopolizing the UI thread.
+        Assert.Equal(
+            DispatcherQueuePriority.Low,
+            RenderPriorityPolicy.PickPriority(lastRenderMs: RenderPriorityPolicy.DefaultFrameBudgetMs + 0.5));
+    }
+
+    [Fact]
+    public void CustomBudget_DemotesPastCustomCeiling()
+    {
+        // A perf-sensitive host can raise or lower the ceiling. The policy
+        // honors the override.
+        Assert.Equal(
+            DispatcherQueuePriority.Low,
+            RenderPriorityPolicy.PickPriority(lastRenderMs: 12, budgetMs: 8));
+
+        Assert.Equal(
+            DispatcherQueuePriority.Normal,
+            RenderPriorityPolicy.PickPriority(lastRenderMs: 12, budgetMs: 32));
+    }
+
+    [Fact]
+    public void DefaultBudget_IsOneFrameAt60Hz()
+    {
+        // Pin the budget so a future "just bump it up" change is a conscious
+        // edit, not a silent regression.
+        Assert.Equal(16.0, RenderPriorityPolicy.DefaultFrameBudgetMs);
+    }
+
+    [Fact]
+    public void ReactorHost_TracksLastRenderMs()
+    {
+        // The wiring contract: ReactorHost stores the most-recent render
+        // duration so RequestRender can consult RenderPriorityPolicy.
+        // Without this field, the policy can't see a slow render and the
+        // PerfStress regression returns.
+        var field = typeof(ReactorHost).GetField("_lastRenderMs",
+            global::System.Reflection.BindingFlags.NonPublic | global::System.Reflection.BindingFlags.Instance);
+        Assert.NotNull(field);
+        Assert.Equal(typeof(double), field!.FieldType);
+    }
+
+    [Fact]
+    public void ReactorHostControl_TracksLastRenderMs()
+    {
+        // Symmetric contract with ReactorHost — embedded hosts must also
+        // demote to Low priority on slow renders.
+        var field = typeof(ReactorHostControl).GetField("_lastRenderMs",
+            global::System.Reflection.BindingFlags.NonPublic | global::System.Reflection.BindingFlags.Instance);
+        Assert.NotNull(field);
+        Assert.Equal(typeof(double), field!.FieldType);
+    }
+}