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>

src/AcDream.App/Rendering/DisplayFramePacingController.cs

@@ -8,7 +8,7 @@ internal interface IDisplayFramePacingSurface
 {
     bool VSync { get; set; }
 
-    int? ActiveMonitorRefreshHz { get; }
+    bool TryGetActiveMonitorRefreshHz(out int refreshHz);
 }
 
 /// <summary>Narrow Silk window adapter for display pacing.</summary>
@@ -27,12 +27,25 @@ internal sealed class SilkDisplayFramePacingSurface : IDisplayFramePacingSurface
         set => _window.VSync = value;
     }
 
-    public int? ActiveMonitorRefreshHz
+    public bool TryGetActiveMonitorRefreshHz(out int refreshHz)
     {
-        get
+        refreshHz = 0;
+        try
         {
-            int? refreshHz = _window.Monitor?.VideoMode.RefreshRate;
+            int? activeRefreshHz = _window.Monitor?.VideoMode.RefreshRate;
-            return refreshHz is > 0 ? refreshHz : null;
+            if (activeRefreshHz is not > 0)
+                return false;
+
+            refreshHz = activeRefreshHz.Value;
+            return true;
+        }
+        catch (Silk.NET.GLFW.GlfwException)
+        {
+            // GLFW can retain a monitor handle while Windows is replacing the
+            // display topology during a console/RDP handoff. Refresh-rate
+            // telemetry is optional; report it as unavailable until the next
+            // move/state event rather than terminating the render host.
+            return false;
         }
     }
 }
@@ -96,7 +109,10 @@ internal sealed class DisplayFramePacingController : IDisposable
 
     public void RefreshActiveMonitor()
     {
-        _activeMonitorRefreshHz = _surface?.ActiveMonitorRefreshHz;
+        _activeMonitorRefreshHz =
+            _surface?.TryGetActiveMonitorRefreshHz(out int refreshHz) == true
+                ? refreshHz
+                : null;
         ApplyResolvedPolicy();
     }
 

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
+            get => _activeMonitorRefreshHz;
-            {
-                RefreshReadCount++;
-                return _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++;
-            {
+            refreshHz = 144;
-                RefreshReadCount++;
+            return true;
-                return 144;
-            }
         }
     }
 }

tests/AcDream.App.Tests/Settings/RuntimeSettingsControllerTests.cs

@@ -1171,13 +1171,16 @@ public sealed class RuntimeSettingsControllerTests
 
         public int? ActiveMonitorRefreshHz
         {
-            get
+            get => _refreshRate;
-            {
-                RefreshReadCount++;
-                return _refreshRate;
-            }
             set => _refreshRate = value;
         }
+
+        public bool TryGetActiveMonitorRefreshHz(out int refreshHz)
+        {
+            RefreshReadCount++;
+            refreshHz = _refreshRate.GetValueOrDefault();
+            return _refreshRate is > 0;
+        }
     }
 
     private sealed class NullKeyboardSource : IKeyboardSource