Window/timing: expose the display refresh rate (AppWindow.RefreshRate)

src/Skyline.Apple/AppKitWindowBackend.cs

@@ -112,6 +112,8 @@ private void UpdateDrawableSize()
         }
     }
 
+    public double RefreshRate => (_window.Screen ?? NSScreen.MainScreen)?.MaximumFramesPerSecond ?? 60;
+
     public string Title
     {
         get => _window.Title;

src/Skyline/AppWindow.cs

@@ -158,6 +158,13 @@ public string? ClipboardText
         set => _backend.ClipboardText = value;
     }
 
+    /// <summary>
+    /// The display's refresh rate in Hz. Pace animation to this. On macOS it
+    /// is the window's screen rate; on GLFW it is the window's monitor rate
+    /// when known (set in fullscreen), otherwise 60.
+    /// </summary>
+    public double RefreshRate => _backend.RefreshRate;
+
     public FrameInfo CurrentFrame => Frame(0);
 
     /// <summary>True when this window has been adopted by an <see cref="AppHost"/>. Hosted windows render on the host's thread, so use <c>AppHost.Run</c>, not <see cref="Run"/>.</summary>

src/Skyline/GlfwWindowBackend.cs

@@ -72,6 +72,9 @@ public unsafe GlfwWindowBackend(AppWindowOptions options)
     public (int Width, int Height) FramebufferSize => (_window.FramebufferSize.X, _window.FramebufferSize.Y);
     public (int Width, int Height) LogicalSize => (_window.Size.X, _window.Size.Y);
 
+    // The window's monitor when GLFW knows it (set in fullscreen), else 60.
+    public double RefreshRate => _window.Monitor?.VideoMode.RefreshRate ?? 60;
+
     public string Title
     {
         get => _window.Title;

src/Skyline/IWindowBackend.cs

@@ -17,6 +17,9 @@ internal interface IWindowBackend : IDisposable
     /// <summary>Window size in logical points.</summary>
     (int Width, int Height) LogicalSize { get; }
 
+    /// <summary>The display's refresh rate in Hz, for pacing animation.</summary>
+    double RefreshRate { get; }
+
     string Title { get; set; }
     string? ClipboardText { get; set; }
 

tests/Skyline.Tests/WindowBackendFactoryTests.cs

@@ -14,6 +14,7 @@ private sealed class FakeBackend : IWindowBackend
     {
         public (int Width, int Height) FramebufferSize => (200, 100);
         public (int Width, int Height) LogicalSize => (100, 50);
+        public double RefreshRate => 60;
         public string Title { get; set; } = "fake";
         public string? ClipboardText { get; set; }
         public bool IsClosing => false;

tests/Skyline.WindowedTests/Program.cs

@@ -38,6 +38,7 @@ void Check(bool condition, string what)
 Check(frame.Dpr > 0, "CurrentFrame has a device pixel ratio");
 Check(Math.Abs(frame.LogicalWidth - frame.PixelWidth / frame.Dpr) < 0.01, "logical width derives from Dpr");
 Check(win.Surface is not null, "native surface source is exposed");
+Check(win.RefreshRate > 0, "RefreshRate reports a positive display rate");
 
 win.ClipboardText = "skyline-test";
 Check(win.ClipboardText == "skyline-test", "clipboard round-trips");
@@ -58,6 +59,7 @@ void Check(bool condition, string what)
     });
     var cf = chromed.CurrentFrame;
     Check(cf.PixelWidth > 0 && cf.PixelHeight > 0, $"{chrome} window constructs with a real surface");
+    Check(chromed.RefreshRate > 0, $"{chrome} GLFW window reports a refresh rate");
 }
 
 var soloResized = false;