initial attempt at detecting chrome double click.

src/Avalonia.Native/DoubleClickHelper.cs

@@ -0,0 +1,31 @@
+using Avalonia.Platform;
+
+namespace Avalonia.Native
+{
+    internal class DoubleClickHelper
+    {
+        private int _clickCount;
+        private Rect _lastClickRect;
+        private ulong _lastClickTime;
+
+        public bool IsDoubleClick(
+            ulong timestamp,
+            Point p)
+        {
+            var settings = AvaloniaLocator.Current.GetService<IPlatformSettings>();
+            var doubleClickTime = settings.DoubleClickTime.TotalMilliseconds;
+
+            if (!_lastClickRect.Contains(p) || timestamp - _lastClickTime > doubleClickTime)
+            {
+                _clickCount = 0;
+            }
+
+            ++_clickCount;
+            _lastClickTime = timestamp;
+            _lastClickRect = new Rect(p, new Size())
+                .Inflate(new Thickness(settings.DoubleClickSize.Width / 2, settings.DoubleClickSize.Height / 2));
+
+            return _clickCount == 2;
+        }
+    }
+}

src/Avalonia.Native/WindowImpl.cs

@@ -1,4 +1,5 @@
 using System;
+using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Platform;
 using Avalonia.Input;
@@ -17,6 +18,8 @@ namespace Avalonia.Native
         private readonly AvaloniaNativePlatformOpenGlInterface _glFeature;
         IAvnWindow _native;
         private double _extendTitleBarHeight = -1;
+        private DoubleClickHelper _doubleClickHelper;
+        
 
         internal WindowImpl(IAvaloniaNativeFactory factory, AvaloniaNativePlatformOptions opts,
             AvaloniaNativePlatformOpenGlInterface glFeature) : base(opts, glFeature)
@@ -24,6 +27,8 @@ namespace Avalonia.Native
             _factory = factory;
             _opts = opts;
             _glFeature = glFeature;
+            _doubleClickHelper = new DoubleClickHelper();
+            
             using (var e = new WindowEvents(this))
             {
                 var context = _opts.UseGpu ? glFeature?.MainContext : null;
@@ -118,7 +123,22 @@ namespace Avalonia.Native
 
                     if(visual == null)
                     {
-                        _native.BeginMoveDrag();
+                        if (_doubleClickHelper.IsDoubleClick(e.Timestamp, e.Position))
+                        {
+                            // TOGGLE WINDOW STATE.
+                            if (WindowState == WindowState.Maximized || WindowState == WindowState.FullScreen)
+                            {
+                                WindowState = WindowState.Normal;
+                            }
+                            else
+                            {
+                                WindowState = WindowState.Maximized;
+                            }
+                        }
+                        else
+                        {
+                            _native.BeginMoveDrag();   
+                        }
                     }
                 }
             }