Pause plugins when covered by other window

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "lib/modio.net"]
+	path = lib/modio.net
+	url = https://github.com/nickelc/modio.net.git

src/DesktopMagic/BuiltInPlugins/BenchmarkPlugin.cs

@@ -9,9 +9,6 @@ namespace DesktopMagic.BuiltInPlugins;
 
 internal class BenchmarkPlugin : Plugin
 {
-    private const int BitmapWidth = 400;
-    private const int BitmapHeight = 300;
-
     private readonly Random rng = new(42);
     private readonly object rngLock = new();
 
@@ -28,7 +25,7 @@ internal class BenchmarkPlugin : Plugin
     [Setting("fps-counter", "Fps")]
     private readonly Label fpsCounter = new("");
 
-    public override int UpdateInterval => 16;
+    public override int UpdateInterval => 1;
 
     public override Bitmap? Main()
     {
@@ -40,8 +37,10 @@ internal class BenchmarkPlugin : Plugin
         }
         lastFrameMs = now;
 
-        int w = BitmapWidth;
-        int h = BitmapHeight;
+        // Render at the full device-pixel window size so both benchmarks cover the
+        // same on-screen area at the same resolution for a fair comparison.
+        int w = Application.WindowPixelSize.Width;
+        int h = Application.WindowPixelSize.Height;
 
         Bitmap bmp = new Bitmap(w, h);
         using Graphics g = Graphics.FromImage(bmp);

src/DesktopMagic/BuiltInPlugins/SkiaBenchmarkPlugin.cs

@@ -4,6 +4,7 @@ using DesktopMagic.Api.Settings;
 using SkiaSharp;
 
 using System;
+using System.Drawing;
 
 namespace DesktopMagic.BuiltInPlugins;
 
@@ -22,9 +23,23 @@ internal class SkiaBenchmarkPlugin : SkiaPlugin
     [Setting("skia-shape-type", "Shape Type")]
     private readonly ComboBox shapeType = new("Rectangles", "Ellipses", "Lines", "Text", "Mixed");
 
-    public override int UpdateInterval => 16;
+    [Setting("skia-fps-counter", "Fps")]
+    private readonly Label fpsCounter = new("");
 
-    public override void Main(SKCanvas canvas)
+    public override int UpdateInterval => 1;
+
+    SKSurface surface;
+
+
+    public override void Start()
+    {
+        Size pixelSize = Application.WindowPixelSize;
+        int w = Math.Max(1, pixelSize.Width);
+        int h = Math.Max(1, pixelSize.Height);
+        surface = SKSurface.Create(new SKImageInfo(w, h, SKColorType.Bgra8888, SKAlphaType.Premul));
+    }
+
+    public override SKImage? Render()
     {
         long now = frameTimer.ElapsedMilliseconds;
         if (lastFrameMs > 0)
@@ -34,8 +49,14 @@ internal class SkiaBenchmarkPlugin : SkiaPlugin
         }
         lastFrameMs = now;
 
-        int w = (int)canvas.DeviceClipBounds.Width;
-        int h = (int)canvas.DeviceClipBounds.Height;
+        // Render at the full device-pixel window size so the benchmark measures
+        // real on-screen resolution instead of being scaled down.
+        Size pixelSize = Application.WindowPixelSize;
+        int w = Math.Max(1, pixelSize.Width);
+        int h = Math.Max(1, pixelSize.Height);
+
+        
+        SKCanvas canvas = surface.Canvas;
 
         canvas.Clear(SKColors.Transparent);
 
@@ -112,12 +133,8 @@ internal class SkiaBenchmarkPlugin : SkiaPlugin
             }
         }
 
-        using var fpsFont = new SKFont(SKTypeface.Default, 14);
-        using var fpsPaint = new SKPaint
-        {
-            Color = color,
-            IsAntialias = false
-        };
-        canvas.DrawText($"FPS: {fps:F1} | Shapes: {count} | Type: {type}", 8, 18, SKTextAlign.Left, fpsFont, fpsPaint);
+        fpsCounter.Value = fps.ToString();
+
+        return surface.Snapshot();
     }
 }

src/DesktopMagic/BuiltInPlugins/TimePlugin.cs

@@ -1,62 +1,56 @@
 using DesktopMagic.Api;
 using DesktopMagic.Api.Settings;
 
+using SkiaSharp;
+
 using System;
-using System.Drawing;
-using System.Drawing.Text;
 
 namespace DesktopMagic.BuiltInPlugins;
 
-internal class TimePlugin : Plugin
+internal class TimePlugin : SkiaPlugin
 {
+    private const float FontSize = 200;
+    private const float ContentPadding = 8;
+
     [Setting("display-seconds", "Show Seconds")]
     private readonly CheckBox displaySecondsCheckBox = new CheckBox(true);
 
-    private string oldTime = string.Empty;
-    private bool themeChanged;
-
     public override int UpdateInterval => 1000;
 
-    public override Bitmap? Main()
+    public override SKImage? Render()
     {
-        string time = displaySecondsCheckBox.Value ? DateTime.Now.ToLongTimeString() : DateTime.Now.ToShortTimeString();
+        string time = GetTime();
 
-        if (oldTime == time && !themeChanged)
+        using SKTypeface typeface = SKTypeface.FromFamilyName(Application.Theme.Font);
+        using SKFont font = new(typeface, FontSize);
+        using SKPaint paint = new()
         {
-            return null;
-        }
+            Color = new SKColor(
+                Application.Theme.PrimaryColor.R,
+                Application.Theme.PrimaryColor.G,
+                Application.Theme.PrimaryColor.B,
+                Application.Theme.PrimaryColor.A),
+            IsAntialias = true
+        };
 
-        oldTime = time;
-        themeChanged = false;
+        float textWidth = font.MeasureText(time, paint);
+        SKFontMetrics metrics = font.Metrics;
+        float textHeight = metrics.Descent - metrics.Ascent;
 
-        using Font font = new Font(Application.Theme.Font, 200);
+        int width = (int)Math.Ceiling(textWidth) + (int)(ContentPadding * 2);
+        int height = (int)Math.Ceiling(textHeight) + (int)(ContentPadding * 2);
 
-        Bitmap bmp = new Bitmap(1, 1);
-        bmp.SetResolution(100, 100);
-        using Graphics tmpGr = Graphics.FromImage(bmp);
-        tmpGr.TextRenderingHint = TextRenderingHint.AntiAlias;
+        using SKSurface surface = SKSurface.Create(new SKImageInfo(width, height, SKColorType.Bgra8888, SKAlphaType.Premul));
+        surface.Canvas.Clear(SKColors.Transparent);
 
-        SizeF size = tmpGr.MeasureString(time, font);
+        // Draw the text at a fixed size into a content-sized image; the window scales it to fit.
+        surface.Canvas.DrawText(time, ContentPadding, -metrics.Ascent + ContentPadding, SKTextAlign.Left, font, paint);
 
-        bmp = new Bitmap((int)size.Width, (int)size.Height);
-        bmp.SetResolution(100, 100);
-
-        using Graphics gr = Graphics.FromImage(bmp);
-        using SolidBrush brush = new SolidBrush(Application.Theme.PrimaryColor);
-
-        gr.TextRenderingHint = TextRenderingHint.AntiAlias;
-        gr.DrawString(time, font, brush, 0, 0);
-
-        return bmp;
+        return surface.Snapshot();
     }
 
-    public override void OnThemeChanged()
+    private string GetTime()
     {
-        themeChanged = true;
+        return displaySecondsCheckBox.Value ? DateTime.Now.ToLongTimeString() : DateTime.Now.ToShortTimeString();
     }
-
-    public override void OnSettingsChanged()
-    {
-        themeChanged = true;
-    }
-}
+}

src/DesktopMagic/Helpers/OcclusionMonitor.cs

@@ -0,0 +1,356 @@
+using System;
+using System.Collections.Generic;
+using System.Drawing;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Windows;
+using System.Windows.Interop;
+
+namespace DesktopMagic.Helpers;
+
+/// <summary>
+/// Periodically checks whether plugin windows are occluded by other top-level windows and pauses
+/// their rendering while they are (a coverage-based pause algorithm, same idea as Lively's).
+/// Pausing stops the plugin update timer, which stops re-rendering and re-uploading frames to the
+/// GPU, so fully covered widgets consume almost no resources.
+/// </summary>
+internal sealed class OcclusionMonitor : IDisposable
+{
+    private const int CheckIntervalMs = 100;
+
+    // Pause only when essentially the whole window is covered so widgets stay responsive even
+    // when a corner is overlapped by another window.
+    private const double CoverageThreshold = 0.95;
+
+    private readonly System.Timers.Timer timer = new(CheckIntervalMs);
+    private readonly Dictionary<PluginWindow, bool> lastPaused = [];
+    private readonly object gate = new();
+    private bool disposed;
+
+    public bool IsRunning { get; private set; }
+
+    public void Start()
+    {
+        if (IsRunning)
+        {
+            return;
+        }
+
+        timer.Elapsed += Timer_Elapsed;
+        timer.Start();
+        IsRunning = true;
+    }
+
+    private void Timer_Elapsed(object? sender, System.Timers.ElapsedEventArgs e)
+    {
+        if (disposed)
+        {
+            return;
+        }
+
+        // System.Timers.Timer can re-enter this handler on another ThreadPool thread when the
+        // previous invocation is still blocked in Dispatcher.Invoke, so serialize the whole pass.
+        if (!Monitor.TryEnter(gate))
+        {
+            return;
+        }
+
+        try
+        {
+            RunOcclusionPass();
+        }
+        finally
+        {
+            Monitor.Exit(gate);
+        }
+    }
+
+    private void RunOcclusionPass()
+    {
+        uint currentProcessId = (uint)Environment.ProcessId;
+
+        // The manager mutates PluginWindows on the UI thread, and the window handle can only be
+        // obtained there, so snapshot (window, handle) pairs over the dispatcher.
+        List<(PluginWindow Window, nint Hwnd)> windows;
+        try
+        {
+            windows = Application.Current.Dispatcher.Invoke(() => Manager.Instance.PluginWindows
+                .OfType<PluginWindow>()
+                .Where(window => window.IsRunning)
+                .Select(window => (Window: window, Hwnd: new WindowInteropHelper(window).Handle))
+                .Where(item => item.Hwnd != nint.Zero)
+                .ToList());
+        }
+        catch
+        {
+            return;
+        }
+
+        HashSet<PluginWindow> live = [.. windows.Select(item => item.Window)];
+
+        foreach ((PluginWindow window, nint hwnd) in windows)
+        {
+            bool paused;
+
+            try
+            {
+                if (!W32.GetWindowRect(hwnd, out W32.RECT windowRect))
+                {
+                    continue;
+                }
+
+                Rectangle windowArea = windowRect;
+                double coverage = ComputeCoverage(windowArea, hwnd, currentProcessId);
+                bool foregroundCovers = IsForegroundCovering(windowArea, hwnd);
+
+                paused = foregroundCovers || coverage >= CoverageThreshold;
+            }
+            catch
+            {
+                continue;
+            }
+
+            if (lastPaused.TryGetValue(window, out bool previous) && previous == paused)
+            {
+                continue;
+            }
+
+            lastPaused[window] = paused;
+
+            try
+            {
+                _ = window.Dispatcher.InvokeAsync(() => window.SetRenderPaused(paused));
+            }
+            catch
+            {
+                // Window is closing; ignore.
+            }
+        }
+
+        // Drop tracking for windows that no longer exist.
+        foreach (PluginWindow stale in lastPaused.Keys.Where(key => !live.Contains(key)).ToArray())
+        {
+            _ = lastPaused.Remove(stale);
+        }
+    }
+
+    /// <summary>
+    /// Computes how much of <paramref name="target"/> (in virtual screen coordinates) is covered by
+    /// visible top-level windows from other processes that are above it in z-order.
+    /// </summary>
+    private static double ComputeCoverage(Rectangle target, nint selfHwnd, uint currentProcessId)
+    {
+        List<Rectangle> coveringRects = [];
+
+        _ = W32.EnumWindows((hwnd, _) =>
+        {
+            if (hwnd == selfHwnd || !W32.IsWindowVisible(hwnd))
+            {
+                return true;
+            }
+
+            // Only windows from other processes occlude our widgets; only non-cloaked windows are
+            // actually visible (exclusive-fullscreen games cloak the desktop).
+            if (W32.GetWindowThreadProcessId(hwnd, out uint processId) == 0
+                || processId == currentProcessId
+                || IsCloaked(hwnd)
+                || !W32.GetWindowRect(hwnd, out W32.RECT rect))
+            {
+                return true;
+            }
+
+            // Explorer's desktop/wallpaper host windows (WorkerW, Progman, ...) report as visible
+            // and span the whole screen but never occlude anything, so ignore them.
+            if (IsDesktopShellWindow(hwnd))
+            {
+                return true;
+            }
+
+            // Fully transparent layered overlays (game overlays etc.) span the whole screen but
+            // cannot be seen, so they never occlude anything.
+            if (IsFullyTransparent(hwnd))
+            {
+                return true;
+            }
+
+            // Ignore windows that are behind ours in z-order.
+            if (!IsWindowAbove(hwnd, selfHwnd))
+            {
+                return true;
+            }
+
+            Rectangle intersection = Rectangle.Intersect(target, rect);
+            if (intersection.Width > 0 && intersection.Height > 0)
+            {
+                coveringRects.Add(intersection);
+            }
+
+            return true;
+        }, nint.Zero);
+
+        if (coveringRects.Count == 0 || target.Width <= 0 || target.Height <= 0)
+        {
+            return 0;
+        }
+
+        return UnionArea(coveringRects) / (double)(target.Width * target.Height);
+    }
+
+    private static bool IsForegroundCovering(Rectangle target, nint selfHwnd)
+    {
+        nint foreground = W32.GetForegroundWindow();
+        if (foreground == nint.Zero || foreground == selfHwnd)
+        {
+            return false;
+        }
+
+        if (W32.GetWindowThreadProcessId(foreground, out _) == 0)
+        {
+            return false;
+        }
+
+        // A maximized/fullscreen app from another process covering the widget pauses it.
+        _ = W32.GetWindowRect(foreground, out W32.RECT rect);
+        Rectangle intersection = Rectangle.Intersect(target, rect);
+
+        return target.Width > 0 && target.Height > 0
+            && intersection.Width >= target.Width * CoverageThreshold
+            && intersection.Height >= target.Height * CoverageThreshold;
+    }
+
+    private static bool IsCloaked(nint hwnd)
+    {
+        return W32.DwmGetWindowAttribute(hwnd, W32.DWMWA_CLOAKED, out int cloaked, sizeof(int)) == 0 && cloaked != 0;
+    }
+
+    /// <summary>
+    /// Returns true for layered windows that are fully transparent (alpha 0), i.e. invisible
+    /// overlay hosts that span the screen but never visibly occlude anything.
+    /// </summary>
+    private static bool IsFullyTransparent(nint hwnd)
+    {
+        if ((W32.GetWindowLong(hwnd, W32.GWL_EXSTYLE) & W32.WS_EX_LAYERED) == 0)
+        {
+            return false;
+        }
+
+        return W32.GetLayeredWindowAttributes(hwnd, out _, out byte alpha, out int flags)
+            && (flags & W32.LWA_ALPHA) != 0
+            && alpha == 0;
+    }
+
+    /// <summary>
+    /// Returns true for the Explorer desktop/wallpaper host windows that span the whole screen but
+    /// never visually occlude anything (wallpaper, hidden desktop icon layer, ...).
+    /// </summary>
+    private static bool IsDesktopShellWindow(nint hwnd)
+    {
+        StringBuilder className = new(256);
+        _ = W32.GetClassName(hwnd, className, className.Capacity);
+
+        return className.ToString() is "WorkerW" or "Progman" or "SHELLDLL_DefView" or "SysListView32";
+    }
+
+    /// <summary>
+    /// Returns true when <paramref name="other"/> is above <paramref name="self"/> in z-order, i.e.
+    /// when walking toward the front (GW_HWNDPREV) from <paramref name="self"/> we reach
+    /// <paramref name="other"/>.
+    /// </summary>
+    private static bool IsWindowAbove(nint other, nint self)
+    {
+        nint current = self;
+        while ((current = W32.GetWindow(current, W32.GW_HWNDPREV)) != nint.Zero)
+        {
+            if (current == other)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /// <summary>
+    /// Computes the total (union) area of <paramref name="rects"/> using a y-scanline sweep so
+    /// overlapping windows are not double counted.
+    /// </summary>
+    private static long UnionArea(List<Rectangle> rects)
+    {
+        if (rects.Count == 0)
+        {
+            return 0;
+        }
+
+        List<int> yEdges = [];
+        foreach (Rectangle rect in rects)
+        {
+            yEdges.Add(rect.Top);
+            yEdges.Add(rect.Bottom);
+        }
+        yEdges.Sort();
+
+        long area = 0;
+        for (int i = 0; i < yEdges.Count - 1; i++)
+        {
+            int y0 = yEdges[i];
+            int y1 = yEdges[i + 1];
+            if (y0 == y1)
+            {
+                continue;
+            }
+
+            // Collect the x extents of all rects that span this band, then merge them.
+            List<(int Left, int Right)> xRanges = [];
+            foreach (Rectangle rect in rects)
+            {
+                if (rect.Top <= y0 && rect.Bottom >= y1)
+                {
+                    xRanges.Add((rect.Left, rect.Right));
+                }
+            }
+
+            if (xRanges.Count == 0)
+            {
+                continue;
+            }
+
+            xRanges.Sort((a, b) => a.Left.CompareTo(b.Left));
+            int mergedLeft = xRanges[0].Left;
+            int mergedRight = xRanges[0].Right;
+            long bandWidth = 0;
+
+            for (int j = 1; j < xRanges.Count; j++)
+            {
+                if (xRanges[j].Left <= mergedRight)
+                {
+                    mergedRight = Math.Max(mergedRight, xRanges[j].Right);
+                }
+                else
+                {
+                    bandWidth += mergedRight - mergedLeft;
+                    mergedLeft = xRanges[j].Left;
+                    mergedRight = xRanges[j].Right;
+                }
+            }
+            bandWidth += mergedRight - mergedLeft;
+
+            area += bandWidth * (y1 - y0);
+        }
+
+        return area;
+    }
+
+    public void Dispose()
+    {
+        if (disposed)
+        {
+            return;
+        }
+
+        disposed = true;
+        timer.Elapsed -= Timer_Elapsed;
+        timer.Stop();
+        timer.Dispose();
+    }
+}

src/DesktopMagic/Helpers/Win32Wrapper.cs

@@ -10,6 +10,18 @@ internal class W32
 
     public const int SM_CYSCREEN = 1;
 
+    public const int DWMWA_CLOAKED = 14;
+
+    public const int GWL_EXSTYLE = -20;
+
+    public const uint GW_HWNDNEXT = 2;
+
+    public const uint GW_HWNDPREV = 3;
+
+    public const int LWA_ALPHA = 0x2;
+
+    public const long WS_EX_LAYERED = 0x00080000L;
+
     public const int SPI_SETDESKWALLPAPER = 20;
     public const int SPIF_SENDWININICHANGE = 0x02;
     public const int SPIF_UPDATEINIFILE = 0x01;
@@ -55,6 +67,26 @@ internal class W32
     [DllImport("dwmapi.dll")]
     public static extern int DwmSetWindowAttribute(nint hwnd, int attr, ref int attrValue, int attrSize);
 
+    [DllImport("dwmapi.dll")]
+    public static extern int DwmGetWindowAttribute(nint hwnd, int dwAttribute, out int pvAttribute, int cbAttribute);
+
+    [DllImport("user32.dll")]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    public static extern bool GetWindowRect(nint hWnd, out RECT lpRect);
+
+    [DllImport("user32.dll")]
+    public static extern int GetWindowLong(nint hWnd, int nIndex);
+
+    [DllImport("user32.dll")]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    public static extern bool GetLayeredWindowAttributes(nint hWnd, out int colorKey, out byte alpha, out int dwFlags);
+
+    [DllImport("user32.dll")]
+    public static extern nint GetWindow(nint hWnd, uint uCmd);
+
+    [DllImport("user32.dll")]
+    public static extern uint GetWindowThreadProcessId(nint hWnd, out uint processId);
+
     [DllImport("user32.dll")]
     [return: MarshalAs(UnmanagedType.Bool)]
     public static extern bool EnumWindows(EnumWindowsProc lpEnumFunc, nint lParam);

src/DesktopMagic/MainWindow.xaml.cs

@@ -19,6 +19,7 @@ public partial class MainWindow : FluentWindow
 {
     private readonly Manager _manager = Manager.Instance;
     private readonly MainWindowDataContext _mainWindowDataContext = new();
+    private OcclusionMonitor? occlusionMonitor;
 
     [Obsolete]
     public MainWindow()
@@ -49,6 +50,10 @@ public partial class MainWindow : FluentWindow
             _manager.LoadPlugins();
             _manager.LoadLayout();
 
+            // Pause plugin rendering while widgets are fully occluded by other windows.
+            occlusionMonitor = new OcclusionMonitor();
+            occlusionMonitor.Start();
+
             if (_manager.Settings.IsFirstRun)
             {
                 _manager.Settings.IsFirstRun = false;
@@ -103,6 +108,9 @@ public partial class MainWindow : FluentWindow
     {
         Microsoft.Win32.SystemEvents.DisplaySettingsChanged -= SystemEvents_DisplaySettingsChanged;
 
+        occlusionMonitor?.Dispose();
+        occlusionMonitor = null;
+
         Visibility = Visibility.Collapsed;
         UpdateLayout();
         _manager.CloseAllPluginWindows();

src/DesktopMagic/Plugins/IPluginWindow.cs

@@ -24,5 +24,10 @@ public interface IPluginWindow
     void ApplySettingValue(string id, string value);
     void ApplyButtonClick(string id);
 
+    /// <summary>
+    /// Pauses or resumes rendering of the plugin window (e.g. when it is fully occluded by other windows).
+    /// </summary>
+    void SetRenderPaused(bool paused);
+
     event System.ComponentModel.CancelEventHandler Closing;
 }

src/DesktopMagic/Plugins/PluginData.cs

@@ -3,6 +3,7 @@
 using DesktopMagic.Api;
 using DesktopMagic.Settings;
 
+using System;
 using System.Drawing;
 
 using Wpf.Ui.Controls;
@@ -12,11 +13,23 @@ namespace DesktopMagic.Plugins;
 internal class PluginData(PluginWindow window, PluginSettings pluginSettings) : IPluginData
 {
     private readonly PluginWindow window = window;
+    private Size windowPixelSize = new(1, 1);
 
     public ITheme Theme => pluginSettings.Theme;
 
     public Size WindowSize => new Size((int)window.ActualWidth, (int)window.ActualHeight);
 
+    public Size WindowPixelSize => windowPixelSize;
+
+    /// <summary>
+    /// Refreshes the cached device-pixel window size. Must be called from the UI thread
+    /// so the value can be read safely by plugins on their render thread.
+    /// </summary>
+    internal void UpdateWindowPixelSize(int width, int height)
+    {
+        windowPixelSize = new Size(Math.Max(1, width), Math.Max(1, height));
+    }
+
     public Point WindowPosition => new Point((int)window.Left, (int)window.Top);
 
     public string PluginName => window.PluginMetadata.Name;

src/DesktopMagic/Plugins/PluginWindow.xaml.cs

@@ -40,12 +40,14 @@ public partial class PluginWindow : Window, IPluginWindow
     private Thread? pluginThread;
     private System.Timers.Timer? updateTimer;
     private Plugin? pluginClassInstance;
+    private PluginData? pluginData;
     private AssemblyLoadContext assemblyLoadContext;
 
     private readonly Rectangle screenBounds;
     private readonly string screenDeviceName;
     private bool isUpdatingPosition = false;
     private bool _editMode = false;
+    private volatile bool renderPaused = false;
 
     private CancellationTokenSource? pluginCancellationTokenSource;
     private FileSystemWatcher? pluginFileWatcher;
@@ -55,6 +57,18 @@ public partial class PluginWindow : Window, IPluginWindow
     private WriteableBitmap? writeableBitmap;
     private BitmapScalingMode lastBitmapScalingMode = BitmapScalingMode.Unspecified;
 
+    // Last presented frame bytes; used to skip re-uploading identical frames so WPF does not
+    // re-upload the texture or re-composite the window (GPU stays idle for static widgets).
+    private byte[]? lastFramePixels;
+    private int lastFrameWidth;
+    private int lastFrameHeight;
+    private int lastFrameStride;
+
+    // Guards against overlapping render ticks. The timer can re-enter while an async
+    // render awaits the UI thread, which would run the plugin concurrently and present
+    // frames out of order. Only one render may be in flight at a time.
+    private long renderInProgress;
+
     // Event handlers on long-lived settings objects, tracked so they can be unsubscribed on close.
     private readonly PropertyChangedEventHandler settingsPropertyChangedHandler;
     private readonly PropertyChangedEventHandler themePropertyChangedHandler;
@@ -334,6 +348,21 @@ public partial class PluginWindow : Window, IPluginWindow
         }
     }
 
+    public void SetRenderPaused(bool paused)
+    {
+        renderPaused = paused;
+
+        if (paused)
+        {
+            updateTimer?.Stop();
+        }
+        else if (IsRunning && pluginClassInstance is { UpdateInterval: > 0 } && updateTimer is not null)
+        {
+            updateTimer.Interval = pluginClassInstance.UpdateInterval;
+            updateTimer.Start();
+        }
+    }
+
     public void UpdatePluginWindow()
     {
         UpdateTimer_Elapsed(updateTimer, null);
@@ -400,8 +429,28 @@ public partial class PluginWindow : Window, IPluginWindow
 
         try
         {
-            Dispatcher.Invoke(() =>
+            int stride = bitmapData.Stride;
+            int height = bitmapData.Height;
+            int byteCount = stride * height;
+
+            unsafe
             {
+                byte* scan0 = (byte*)bitmapData.Scan0.ToPointer();
+
+                // Skip the present when the frame is byte-identical to the last presented one so WPF
+                // does not re-upload the texture or re-composite the window (GPU stays idle for
+                // static widgets that keep ticking).
+                if (lastFramePixels is { Length: > 0 }
+                    && lastFrameWidth == bitmapData.Width
+                    && lastFrameHeight == height
+                    && lastFrameStride == stride
+                    && lastFramePixels.AsSpan().SequenceEqual(new ReadOnlySpan<byte>(scan0, byteCount)))
+                {
+                    return;
+                }
+
+                Dispatcher.Invoke(() =>
+                {
                 if (lastBitmapScalingMode != scalingMode)
                 {
                     RenderOptions.SetBitmapScalingMode(image, scalingMode);
@@ -410,9 +459,19 @@ public partial class PluginWindow : Window, IPluginWindow
 
                 if (writeableBitmap == null || writeableBitmap.PixelWidth != bitmapData.Width || writeableBitmap.PixelHeight != bitmapData.Height)
                 {
+                    // A frame sized to the window maps 1:1 to the display when the bitmap uses the
+                    // actual monitor DPI, so WPF skips the (expensive) per-frame scaling filter.
+                    // Smaller content-sized frames keep 96 DPI to preserve their on-screen semantics.
+                    DpiScale dpi = VisualTreeHelper.GetDpi(this);
+                    bool fullWindowFrame = pluginData != null
+                        && bitmapData.Width == pluginData.WindowPixelSize.Width
+                        && bitmapData.Height == pluginData.WindowPixelSize.Height;
+                    double dpiX = fullWindowFrame ? dpi.DpiScaleX * 96 : bitmap.HorizontalResolution;
+                    double dpiY = fullWindowFrame ? dpi.DpiScaleY * 96 : bitmap.VerticalResolution;
+
                     writeableBitmap = new WriteableBitmap(
                         bitmapData.Width, bitmapData.Height,
-                        bitmap.HorizontalResolution, bitmap.VerticalResolution,
+                        dpiX, dpiY,
                         PixelFormats.Bgra32, null);
                     image.Source = writeableBitmap;
                 }
@@ -434,7 +493,17 @@ public partial class PluginWindow : Window, IPluginWindow
                 {
                     writeableBitmap.Unlock();
                 }
-            });
+                });
+
+                if (lastFramePixels is null || lastFramePixels.Length != byteCount)
+                {
+                    lastFramePixels = new byte[byteCount];
+                }
+                lastFrameWidth = bitmapData.Width;
+                lastFrameHeight = height;
+                lastFrameStride = stride;
+                new ReadOnlySpan<byte>(scan0, byteCount).CopyTo(lastFramePixels);
+            }
         }
         finally
         {
@@ -560,7 +629,8 @@ public partial class PluginWindow : Window, IPluginWindow
         if (instance is Plugin plugin)
         {
             pluginClassInstance = plugin;
-            pluginClassInstance.Application = new PluginData(this, settings);
+            pluginData = new PluginData(this, settings);
+            pluginClassInstance.Application = pluginData;
         }
         else
         {
@@ -589,6 +659,8 @@ public partial class PluginWindow : Window, IPluginWindow
         };
         updateTimer.Elapsed += UpdateTimer_Elapsed;
 
+        await Dispatcher.InvokeAsync(RefreshWindowPixelSize);
+
         pluginClassInstance.Start();
         if (pluginClassInstance is AsyncPlugin asyncPluginStart)
         {
@@ -599,7 +671,7 @@ public partial class PluginWindow : Window, IPluginWindow
         UpdatePluginWindow();
         await Dispatcher.InvokeAsync(ThemeChanged);
 
-        if (pluginClassInstance.UpdateInterval > 0)
+        if (pluginClassInstance.UpdateInterval > 0 && !renderPaused)
         {
             updateTimer.Interval = pluginClassInstance.UpdateInterval;
             updateTimer.Start();
@@ -963,6 +1035,13 @@ public partial class PluginWindow : Window, IPluginWindow
             return;
         }
 
+        // Drop the tick when a render is still in flight instead of running the plugin
+        // concurrently (the timer can re-enter while the async render awaits the UI thread).
+        if (Interlocked.CompareExchange(ref renderInProgress, 1, 0) != 0)
+        {
+            return;
+        }
+
         try
         {
             if (plugin is SkiaPlugin or SkiaAsyncPlugin)
@@ -1034,6 +1113,10 @@ public partial class PluginWindow : Window, IPluginWindow
             });
             Exit();
         }
+        finally
+        {
+            Interlocked.Exchange(ref renderInProgress, 0);
+        }
     }
 
     private async Task RenderSkiaFrame()
@@ -1045,62 +1128,128 @@ public partial class PluginWindow : Window, IPluginWindow
             return;
         }
 
-        int width = 0;
-        int height = 0;
-
-        await Dispatcher.InvokeAsync(() =>
-        {
-            width = Math.Max(1, (int)ActualWidth);
-            height = Math.Max(1, (int)ActualHeight);
-        });
-
-        using SKSurface surface = SKSurface.Create(new SKImageInfo(width, height, SKColorType.Bgra8888, SKAlphaType.Premul));
-        surface.Canvas.Clear(SKColors.Transparent);
+        SKImage? skImage;
 
         if (plugin is SkiaAsyncPlugin skiaAsyncPlugin)
         {
             CancellationToken token = pluginCancellationTokenSource?.Token ?? CancellationToken.None;
-            await skiaAsyncPlugin.MainAsync(surface.Canvas, token);
+            skImage = await skiaAsyncPlugin.RenderAsync(token);
         }
         else if (plugin is SkiaPlugin skiaPlugin)
         {
-            skiaPlugin.Main(surface.Canvas);
+            skImage = skiaPlugin.Render();
         }
-
-        SKPixmap? pixmap = surface.PeekPixels();
-
-        if (pixmap is null)
+        else
         {
             return;
         }
 
-        await Dispatcher.InvokeAsync(() =>
+        // Null keeps the previous frame, matching how GDI plugins return null from Main().
+        if (skImage is null)
         {
-            if (writeableBitmap is null || writeableBitmap.PixelWidth != width || writeableBitmap.PixelHeight != height)
+            return;
+        }
+
+        using (skImage)
+        {
+            using SKImage raster = skImage.ToRasterImage();
+            using SKPixmap? pixmap = raster.PeekPixels();
+
+            if (pixmap is null)
             {
-                writeableBitmap = new WriteableBitmap(width, height, 96, 96, PixelFormats.Bgra32, null);
-                image.Source = writeableBitmap;
+                return;
             }
 
-            writeableBitmap.Lock();
-            try
+            int width = pixmap.Width;
+            int height = pixmap.Height;
+            int byteCount = pixmap.RowBytes * height;
+
+            // Skip the present when the frame is byte-identical to the last presented one so WPF
+            // does not re-upload the texture or re-composite the window (GPU stays idle for
+            // static widgets that keep ticking).
+            if (lastFramePixels is { Length: > 0 }
+                && lastFrameWidth == width
+                && lastFrameHeight == height
+                && lastFrameStride == pixmap.RowBytes
+                && lastFramePixels.AsSpan().SequenceEqual(pixmap.GetPixelSpan()))
             {
-                unsafe
+                return;
+            }
+
+            BitmapScalingMode scalingMode = plugin.RenderQuality switch
+            {
+                RenderQuality.High => BitmapScalingMode.HighQuality,
+                RenderQuality.Low => BitmapScalingMode.LowQuality,
+                RenderQuality.Performance => BitmapScalingMode.NearestNeighbor,
+                _ => BitmapScalingMode.Unspecified
+            };
+
+            await Dispatcher.InvokeAsync(() =>
+            {
+                if (lastBitmapScalingMode != scalingMode)
                 {
-                    Buffer.MemoryCopy(
-                        pixmap.GetPixels().ToPointer(),
-                        writeableBitmap.BackBuffer.ToPointer(),
-                        writeableBitmap.BackBufferStride * height,
-                        pixmap.RowBytes * pixmap.Height);
+                    RenderOptions.SetBitmapScalingMode(image, scalingMode);
+                    lastBitmapScalingMode = scalingMode;
                 }
 
-                writeableBitmap.AddDirtyRect(new Int32Rect(0, 0, width, height));
-            }
-            finally
+                if (writeableBitmap is null || writeableBitmap.PixelWidth != width || writeableBitmap.PixelHeight != height)
+                {
+                    // A frame sized to the window maps 1:1 to the display when the bitmap uses the
+                    // actual monitor DPI, so WPF skips the (expensive) per-frame scaling filter.
+                    // Smaller content-sized frames keep 96 DPI to preserve their on-screen semantics.
+                    DpiScale dpi = VisualTreeHelper.GetDpi(this);
+                    bool fullWindowFrame = pluginData != null
+                        && width == pluginData.WindowPixelSize.Width
+                        && height == pluginData.WindowPixelSize.Height;
+                    double dpiX = fullWindowFrame ? dpi.DpiScaleX * 96 : 96;
+                    double dpiY = fullWindowFrame ? dpi.DpiScaleY * 96 : 96;
+
+                    // Skia outputs premultiplied alpha, so use Pbgra32 (premultiplied BGRA).
+                    // Bgra32 is straight alpha and would render semi-transparent pixels too dark.
+                    writeableBitmap = new WriteableBitmap(width, height, dpiX, dpiY, PixelFormats.Pbgra32, null);
+                    image.Source = writeableBitmap;
+                }
+
+                writeableBitmap.Lock();
+                try
+                {
+                    int destStride = writeableBitmap.BackBufferStride;
+                    int srcStride = pixmap.RowBytes;
+                    int copyWidth = Math.Min(srcStride, destStride);
+                    int copyRows = Math.Min(pixmap.Height, height);
+
+                    unsafe
+                    {
+                        byte* src = (byte*)pixmap.GetPixels().ToPointer();
+                        byte* dst = (byte*)writeableBitmap.BackBuffer.ToPointer();
+
+                        for (int row = 0; row < copyRows; row++)
+                        {
+                            Buffer.MemoryCopy(
+                                src + (row * srcStride),
+                                dst + (row * destStride),
+                                destStride,
+                                copyWidth);
+                        }
+                    }
+
+                    writeableBitmap.AddDirtyRect(new Int32Rect(0, 0, width, height));
+                }
+                finally
+                {
+                    writeableBitmap.Unlock();
+                }
+            });
+
+            if (lastFramePixels is null || lastFramePixels.Length != byteCount)
             {
-                writeableBitmap.Unlock();
+                lastFramePixels = new byte[byteCount];
             }
-        });
+            lastFrameWidth = width;
+            lastFrameHeight = height;
+            lastFrameStride = pixmap.RowBytes;
+            pixmap.GetPixelSpan().CopyTo(lastFramePixels);
+        }
     }
 
     private void Border_SizeChanged(object sender, SizeChangedEventArgs e)
@@ -1257,6 +1406,7 @@ public partial class PluginWindow : Window, IPluginWindow
         }
 
         settings.Position = ScreenUtilities.PositionToPercent(new System.Windows.Point(Left, Top), screenBounds);
+        RefreshWindowPixelSize();
     }
 
     private void Window_SizeChanged(object sender, SizeChangedEventArgs e)
@@ -1267,6 +1417,13 @@ public partial class PluginWindow : Window, IPluginWindow
         }
 
         tileBar.CaptionHeight = ActualHeight - 10;
+        RefreshWindowPixelSize();
+    }
+
+    private void RefreshWindowPixelSize()
+    {
+        DpiScale dpi = VisualTreeHelper.GetDpi(this);
+        pluginData?.UpdateWindowPixelSize((int)Math.Ceiling(ActualWidth * dpi.DpiScaleX), (int)Math.Ceiling(ActualHeight * dpi.DpiScaleY));
     }
 
     private void Image_MouseDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
@@ -1275,8 +1432,15 @@ public partial class PluginWindow : Window, IPluginWindow
 
         ImageSource imageSource = image.Source;
         BitmapSource bitmapImage = (BitmapSource)imageSource;
-        double pixelMousePositionX = e.GetPosition(image).X * bitmapImage.PixelWidth / image.ActualHeight;
-        double pixelMousePositionY = e.GetPosition(image).Y * bitmapImage.PixelHeight / image.ActualHeight;
+        double actualWidth = image.ActualWidth;
+        double actualHeight = image.ActualHeight;
+        if (actualWidth <= 0 || actualHeight <= 0)
+        {
+            return;
+        }
+
+        double pixelMousePositionX = e.GetPosition(image).X * bitmapImage.PixelWidth / actualWidth;
+        double pixelMousePositionY = e.GetPosition(image).Y * bitmapImage.PixelHeight / actualHeight;
 
         MouseButton mouseButton;
 
@@ -1306,8 +1470,15 @@ public partial class PluginWindow : Window, IPluginWindow
     {
         ImageSource imageSource = image.Source;
         BitmapSource bitmapImage = (BitmapSource)imageSource;
-        double pixelMousePositionX = e.GetPosition(image).X * bitmapImage.PixelWidth / image.ActualHeight;
-        double pixelMousePositionY = e.GetPosition(image).Y * bitmapImage.PixelHeight / image.ActualHeight;
+        double actualWidth = image.ActualWidth;
+        double actualHeight = image.ActualHeight;
+        if (actualWidth <= 0 || actualHeight <= 0)
+        {
+            return;
+        }
+
+        double pixelMousePositionX = e.GetPosition(image).X * bitmapImage.PixelWidth / actualWidth;
+        double pixelMousePositionY = e.GetPosition(image).Y * bitmapImage.PixelHeight / actualHeight;
 
         System.Drawing.Point point = new System.Drawing.Point((int)pixelMousePositionX, (int)pixelMousePositionY);
         pluginClassInstance?.OnMouseMove(point);
@@ -1317,8 +1488,15 @@ public partial class PluginWindow : Window, IPluginWindow
     {
         ImageSource imageSource = image.Source;
         BitmapSource bitmapImage = (BitmapSource)imageSource;
-        double pixelMousePositionX = e.GetPosition(image).X * bitmapImage.PixelWidth / image.ActualHeight;
-        double pixelMousePositionY = e.GetPosition(image).Y * bitmapImage.PixelHeight / image.ActualHeight;
+        double actualWidth = image.ActualWidth;
+        double actualHeight = image.ActualHeight;
+        if (actualWidth <= 0 || actualHeight <= 0)
+        {
+            return;
+        }
+
+        double pixelMousePositionX = e.GetPosition(image).X * bitmapImage.PixelWidth / actualWidth;
+        double pixelMousePositionY = e.GetPosition(image).Y * bitmapImage.PixelHeight / actualHeight;
 
         System.Drawing.Point point = new System.Drawing.Point((int)pixelMousePositionX, (int)pixelMousePositionY);
         pluginClassInstance?.OnMouseWheel(point, e.Delta);

src/DesktopMagic/Plugins/WebPluginWindow.xaml.cs

@@ -194,6 +194,23 @@ public partial class WebPluginWindow : Window, IPluginWindow
         }
     }
 
+    public void SetRenderPaused(bool paused)
+    {
+        if (webView.CoreWebView2 is null)
+        {
+            return;
+        }
+
+        if (paused)
+        {
+            _ = webView.CoreWebView2.TrySuspendAsync();
+        }
+        else
+        {
+            webView.CoreWebView2.Resume();
+        }
+    }
+
     protected override void OnSourceInitialized(EventArgs e)
     {
         base.OnSourceInitialized(e);

src/DesktopMagicPluginAPI/IPluginData.cs

@@ -13,10 +13,16 @@ public interface IPluginData
     ITheme Theme { get; }
 
     /// <summary>
-    /// Gets the window size of the plugin window.
+    /// Gets the window size of the plugin window in device independent pixels.
     /// </summary>
     Size WindowSize { get; }
 
+    /// <summary>
+    /// Gets the window size of the plugin window in physical (device) pixels.
+    /// Use this to create a rendering surface that maps 1:1 to the display.
+    /// </summary>
+    Size WindowPixelSize { get; }
+
     /// <summary>
     /// Gets the window position of the plugin window.
     /// </summary>

src/DesktopMagicPluginAPI/SkiaAsyncPlugin.cs

@@ -8,14 +8,17 @@ namespace DesktopMagic.Api;
 
 /// <summary>
 /// Provides an abstract base class for creating asynchronous plugins using SkiaSharp rendering.
-/// Plugins override <see cref="MainAsync(SKCanvas, CancellationToken)"/> instead of <see cref="Plugin.Main()"/>.
+/// Plugins override <see cref="RenderAsync(CancellationToken)"/> and return an <see cref="SKImage"/>
+/// of their own size, just like <see cref="AsyncPlugin.MainAsync(CancellationToken)"/> returns a
+/// <see cref="Bitmap"/>. The returned image is scaled to fit the plugin window while preserving
+/// its aspect ratio.
 /// All existing <see cref="Plugin"/>, <see cref="AsyncPlugin"/>, and <see cref="SkiaPlugin"/> code is unaffected.
 /// </summary>
 public abstract class SkiaAsyncPlugin : AsyncPlugin
 {
     /// <summary>
     /// This method is sealed and returns null for async Skia plugins.
-    /// Override <see cref="MainAsync(SKCanvas, CancellationToken)"/> instead.
+    /// Override <see cref="RenderAsync(CancellationToken)"/> instead.
     /// </summary>
     public sealed override Task<Bitmap?> MainAsync(CancellationToken cancellationToken)
     {
@@ -24,8 +27,9 @@ public abstract class SkiaAsyncPlugin : AsyncPlugin
 
     /// <summary>
     /// Called every <see cref="Plugin.UpdateInterval"/> milliseconds to render the plugin asynchronously.
+    /// Return <see langword="null"/> to keep the previously rendered image.
     /// </summary>
-    /// <param name="canvas">The SkiaSharp canvas to draw on.</param>
     /// <param name="cancellationToken">Token signaled when the host requests cancellation.</param>
-    public abstract Task MainAsync(SKCanvas canvas, CancellationToken cancellationToken);
+    /// <returns>The rendered image, or <see langword="null"/> to keep the previous frame.</returns>
+    public abstract Task<SKImage?> RenderAsync(CancellationToken cancellationToken);
 }

src/DesktopMagicPluginAPI/SkiaPlugin.cs

@@ -6,14 +6,16 @@ namespace DesktopMagic.Api;
 
 /// <summary>
 /// Provides an abstract base class for creating plugins using SkiaSharp rendering.
-/// Plugins override <see cref="Main(SKCanvas)"/> instead of <see cref="Plugin.Main()"/>.
+/// Plugins override <see cref="Render()"/> and return an <see cref="SKImage"/> of their own size,
+/// just like <see cref="Plugin.Main()"/> returns a <see cref="Bitmap"/>. The returned image is
+/// scaled to fit the plugin window while preserving its aspect ratio.
 /// All existing <see cref="Plugin"/> and <see cref="AsyncPlugin"/> code is unaffected.
 /// </summary>
 public abstract class SkiaPlugin : Plugin
 {
     /// <summary>
     /// This method is sealed and returns null for Skia plugins.
-    /// Override <see cref="Main(SKCanvas)"/> instead.
+    /// Override <see cref="Render()"/> instead.
     /// </summary>
     /// <returns>Always null.</returns>
     public sealed override Bitmap? Main()
@@ -23,7 +25,8 @@ public abstract class SkiaPlugin : Plugin
 
     /// <summary>
     /// Called every <see cref="Plugin.UpdateInterval"/> milliseconds to render the plugin.
+    /// Return <see langword="null"/> to keep the previously rendered image.
     /// </summary>
-    /// <param name="canvas">The SkiaSharp canvas to draw on.</param>
-    public abstract void Main(SKCanvas canvas);
+    /// <returns>The rendered image, or <see langword="null"/> to keep the previous frame.</returns>
+    public abstract SKImage? Render();
 }