fix(rendering): survive display topology handoff

Model active-monitor refresh telemetry as optional so GLFW display replacement during console/RDP transitions cannot escape the native move callback and terminate the client. Preserve the existing software pacing fallback until a later monitor event succeeds.

Co-authored-by: OpenAI Codex <codex@openai.com>

tests/AcDream.App.Tests/Rendering/DisplayFramePacingControllerTests.cs

@@ -98,6 +98,37 @@ public sealed class DisplayFramePacingControllerTests
         Assert.Equal(new FramePacingPolicy(false, 144d), controller.Policy);
     }
 
+    [Fact]
+    public void Unavailable_monitor_during_display_handoff_uses_fallback_policy()
+    {
+        using var profiler = new FrameProfiler();
+        using var controller = new DisplayFramePacingController(
+            uncappedRendering: false,
+            profiler,
+            new FramePacingController(
+                new FakeClock(1_000),
+                new RecordingWaiter()));
+        var surface = new FakeSurface
+        {
+            VSync = false,
+            ActiveMonitorRefreshHz = 144,
+        };
+        controller.InitializeStartup(requestedVSync: false);
+        controller.BindSurface(surface);
+        controller.RefreshActiveMonitor();
+        Assert.Equal(new FramePacingPolicy(false, 144d), controller.Policy);
+
+        surface.MonitorAvailable = false;
+        controller.OnWindowMoved(default);
+
+        Assert.Equal(
+            new FramePacingPolicy(
+                false,
+                FramePacingPolicy.FallbackRefreshHz),
+            controller.Policy);
+        Assert.False(surface.VSync);
+    }
+
     [Fact]
     public void Explicit_uncapped_mode_disables_both_pacing_mechanisms()
     {
@@ -172,16 +203,22 @@ public sealed class DisplayFramePacingControllerTests
 
         public int RefreshReadCount { get; private set; }
 
+        public bool MonitorAvailable { get; set; } = true;
+
         public int? ActiveMonitorRefreshHz
         {
-            get
-            {
-                RefreshReadCount++;
-                return _activeMonitorRefreshHz;
-            }
+            get => _activeMonitorRefreshHz;
             set => _activeMonitorRefreshHz = value;
         }
 
+        public bool TryGetActiveMonitorRefreshHz(out int refreshHz)
+        {
+            RefreshReadCount++;
+            refreshHz = _activeMonitorRefreshHz.GetValueOrDefault();
+            return MonitorAvailable &&
+                _activeMonitorRefreshHz is > 0;
+        }
+
         public void Dispose() => Disposed = true;
     }
 

tests/AcDream.App.Tests/Rendering/SilkWindowCallbackBindingTests.cs

@@ -789,13 +789,11 @@ public sealed class SilkWindowCallbackBindingTests
         public bool VSync { get; set; }
         public int RefreshReadCount { get; private set; }
 
-        public int? ActiveMonitorRefreshHz
+        public bool TryGetActiveMonitorRefreshHz(out int refreshHz)
         {
-            get
-            {
-                RefreshReadCount++;
-                return 144;
-            }
+            RefreshReadCount++;
+            refreshHz = 144;
+            return true;
         }
     }
 }