Respect OS "reduce motion" setting for Avalonia UI animations (#4904)

Author: GabrielDuf

src/UniGetUI.Avalonia/Infrastructure/DirectionalSlideTransition.cs

@@ -29,6 +29,14 @@ public sealed class DirectionalSlideTransition : IPageTransition
 
     public async Task Start(Visual? from, Visual? to, bool forward, CancellationToken cancellationToken)
     {
+        // Honor the OS "reduce motion" preference: swap pages instantly, no slide.
+        if (MotionPreference.ReducedMotion)
+        {
+            if (from is not null) { from.IsVisible = false; from.RenderTransform = null; }
+            to?.RenderTransform = null;
+            return;
+        }
+
         double sign = Reverse ? -1d : 1d;
         double width = (to ?? from)?.GetVisualParent()?.Bounds.Width
                        ?? (to ?? from)?.Bounds.Width ?? 0d;

src/UniGetUI.Avalonia/Infrastructure/EntrancePageTransition.cs

@@ -22,6 +22,14 @@ public sealed class EntrancePageTransition : IPageTransition
 
     public async Task Start(Visual? from, Visual? to, bool forward, CancellationToken cancellationToken)
     {
+        // Honor "reduce motion": hide the outgoing page and show the incoming one instantly, no overlap.
+        if (MotionPreference.ReducedMotion)
+        {
+            from?.Opacity = 0;
+            to?.Opacity = 1;
+            return;
+        }
+
         // Drop the outgoing page immediately so only the incoming page animates in.
         from?.Opacity = 0;
 

src/UniGetUI.Avalonia/Infrastructure/MotionPreference.cs

@@ -0,0 +1,103 @@
+using System;
+using System.Diagnostics;
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+using UniGetUI.Core.Logging;
+
+namespace UniGetUI.Avalonia.Infrastructure;
+
+/// <summary>
+/// Reads the OS "reduce motion / show animations" accessibility preference so UI
+/// transitions can honor it (Windows: Settings → Accessibility → Visual effects →
+/// Animation effects; macOS: Accessibility → Display → Reduce motion). Windows is read
+/// live on each access (cheap P/Invoke); macOS/Linux are read once and cached because
+/// they spawn a process. Defaults to "animations on" if the preference can't be read.
+/// </summary>
+internal static class MotionPreference
+{
+    private static bool? _cachedUnix;
+
+    /// <summary>True when the user has asked the OS to minimize animations.</summary>
+    public static bool ReducedMotion
+    {
+        get
+        {
+            if (OperatingSystem.IsWindows())
+                return GetWindowsReducedMotion();
+            return _cachedUnix ??= GetUnixReducedMotion();
+        }
+    }
+
+    [SupportedOSPlatform("windows")]
+    private static bool GetWindowsReducedMotion()
+    {
+        try
+        {
+            // Same signal WinUI's UISettings.AnimationsEnabled reads; pvParam is a BOOL.
+            int enabled = 1;
+            if (NativeMethods.SystemParametersInfoW(NativeMethods.SPI_GETCLIENTAREAANIMATION, 0, ref enabled, 0))
+                return enabled == 0;
+        }
+        catch (Exception ex)
+        {
+            Logger.Warn("Could not read SPI_GETCLIENTAREAANIMATION; assuming animations enabled");
+            Logger.Warn(ex);
+        }
+        return false;
+    }
+
+    private static bool GetUnixReducedMotion()
+    {
+        try
+        {
+            if (OperatingSystem.IsMacOS())
+                return ReadCommand("/usr/bin/defaults", ["read", "com.apple.universalaccess", "reduceMotion"]) is "1";
+
+            if (OperatingSystem.IsLinux())
+                return string.Equals(
+                    ReadCommand("/usr/bin/gsettings", ["get", "org.gnome.desktop.interface", "gtk-enable-animations"]),
+                    "false", StringComparison.OrdinalIgnoreCase);
+        }
+        catch (Exception ex)
+        {
+            Logger.Warn("Could not read OS reduce-motion preference; assuming animations enabled");
+            Logger.Warn(ex);
+        }
+        return false;
+    }
+
+    private static string? ReadCommand(string fileName, string[] args)
+    {
+        var psi = new ProcessStartInfo
+        {
+            FileName = fileName,
+            UseShellExecute = false,
+            RedirectStandardOutput = true,
+            RedirectStandardError = true,
+            CreateNoWindow = true,
+        };
+        foreach (var arg in args)
+            psi.ArgumentList.Add(arg);
+
+        using var proc = Process.Start(psi);
+        if (proc is null)
+            return null;
+
+        string output = proc.StandardOutput.ReadToEnd();
+        if (!proc.WaitForExit(2000))
+        {
+            try { proc.Kill(); } catch { /* best effort */ }
+            return null;
+        }
+        return proc.ExitCode == 0 ? output.Trim() : null;
+    }
+
+    private static class NativeMethods
+    {
+        public const uint SPI_GETCLIENTAREAANIMATION = 0x1042;
+
+        [DllImport("user32.dll", SetLastError = true)]
+        [return: MarshalAs(UnmanagedType.Bool)]
+        public static extern bool SystemParametersInfoW(uint uiAction, uint uiParam, ref int pvParam, uint fWinIni);
+    }
+}

src/UniGetUI.Avalonia/Views/DialogPages/PackageDetailsWindow.axaml.cs

@@ -45,6 +45,10 @@ public PackageDetailsWindow(IPackage package, OperationType operation)
         InitializeComponent();
         UniGetUI.Avalonia.Infrastructure.MicaWindowHelper.Apply(this);
 
+        // Honor the OS "reduce motion" preference: drop the screenshot slide animation.
+        if (MotionPreference.ReducedMotion)
+            ScreenshotsCarousel.PageTransition = null;
+
         _vm.CloseRequested += (_, _) => Close();
         _vm.DetailsLoaded += (_, _) =>
         {