Convert max window size to client size.

src/Gtk/Perspex.Gtk/WindowImpl.cs

@@ -57,11 +57,12 @@ namespace Perspex.Gtk
             set;
         }
 
-        public Size MaxWindowSize
+        public Size MaxClientSize
         {
             get
             {
-                // TODO: This should take into account things such as taskbar etc.
+                // TODO: This should take into account things such as taskbar and window border
+                // thickness etc.
                 return new Size(Screen.Width, Screen.Height);
             }
         }

src/Perspex.Controls/Platform/IWindowImpl.cs

@@ -13,7 +13,7 @@ namespace Perspex.Platform
         /// <summary>
         /// Gets the maximum size of a window on the system.
         /// </summary>
-        Size MaxWindowSize { get; }
+        Size MaxClientSize { get; }
 
         /// <summary>
         /// Sets the title of the window.

src/Perspex.Controls/Window.cs

@@ -57,7 +57,7 @@ namespace Perspex.Controls
 
         private object _dialogResult;
 
-        private Size _maxPlatformWindowSize;
+        private Size _maxPlatformClientSize;
 
         /// <summary>
         /// Initializes static members of the <see cref="Window"/> class.
@@ -74,7 +74,7 @@ namespace Perspex.Controls
         public Window()
             : base(Locator.Current.GetService<IWindowImpl>())
         {
-            _maxPlatformWindowSize = this.PlatformImpl.MaxWindowSize;
+            _maxPlatformClientSize = this.PlatformImpl.MaxClientSize;
         }
 
         /// <summary>
@@ -196,7 +196,7 @@ namespace Perspex.Controls
         {
             var sizeToContent = SizeToContent;
             var size = ClientSize;
-            var desired = base.MeasureOverride(availableSize.Constrain(_maxPlatformWindowSize));
+            var desired = base.MeasureOverride(availableSize.Constrain(_maxPlatformClientSize));
 
             switch (sizeToContent)
             {

src/Perspex.SceneGraph/Thickness.cs

@@ -138,6 +138,32 @@ namespace Perspex
                 a.Bottom + b.Bottom);
         }
 
+        /// <summary>
+        /// Adds a Thickness to a Size.
+        /// </summary>
+        /// <param name="size">The size.</param>
+        /// <param name="thickness">The thickness.</param>
+        /// <returns>The equality.</returns>
+        public static Size operator +(Size size, Thickness thickness)
+        {
+            return new Size(
+                size.Width + thickness.Left + thickness.Right,
+                size.Height + thickness.Top + thickness.Bottom);
+        }
+
+        /// <summary>
+        /// Subtracts a Thickness from a Size.
+        /// </summary>
+        /// <param name="size">The size.</param>
+        /// <param name="thickness">The thickness.</param>
+        /// <returns>The equality.</returns>
+        public static Size operator -(Size size, Thickness thickness)
+        {
+            return new Size(
+                size.Width - (thickness.Left + thickness.Right),
+                size.Height - (thickness.Top + thickness.Bottom));
+        }
+
         /// <summary>
         /// Parses a <see cref="Thickness"/> string.
         /// </summary>

src/Windows/Perspex.Win32/WindowImpl.cs

@@ -53,6 +53,25 @@ namespace Perspex.Win32
 
         public Action<Size> Resized { get; set; }
 
+        public Thickness BorderThickness
+        {
+            get
+            {
+                var style = UnmanagedMethods.GetWindowLong(_hwnd, -16);
+                var exStyle = UnmanagedMethods.GetWindowLong(_hwnd, -20);
+                var padding = new UnmanagedMethods.RECT();
+
+                if (UnmanagedMethods.AdjustWindowRectEx(ref padding, style, false, exStyle))
+                {
+                    return new Thickness(-padding.left, -padding.top, padding.right, padding.bottom);
+                }
+                else
+                {
+                    throw new Win32Exception();
+                }
+            }
+        }
+
         public Size ClientSize
         {
             get
@@ -66,21 +85,16 @@ namespace Perspex.Win32
             {
                 if (value != ClientSize)
                 {
-                    var style = UnmanagedMethods.GetWindowLong(_hwnd, -16);
-                    var exStyle = UnmanagedMethods.GetWindowLong(_hwnd, -20);
-                    var padding = new UnmanagedMethods.RECT();
-
-                    if (UnmanagedMethods.AdjustWindowRectEx(ref padding, style, false, exStyle))
-                    {
-                        UnmanagedMethods.SetWindowPos(
-                            _hwnd,
-                            IntPtr.Zero,
-                            0,
-                            0,
-                            -padding.left + padding.right + (int)value.Width,
-                            -padding.top + padding.bottom + (int)value.Height,
-                            UnmanagedMethods.SetWindowPosFlags.SWP_RESIZE);
-                    }
+                    value += BorderThickness;
+
+                    UnmanagedMethods.SetWindowPos(
+                        _hwnd,
+                        IntPtr.Zero,
+                        0,
+                        0,
+                        (int)value.Width,
+                        (int)value.Height,
+                        UnmanagedMethods.SetWindowPosFlags.SWP_RESIZE);
                 }
             }
         }
@@ -97,13 +111,14 @@ namespace Perspex.Win32
             set { UnmanagedMethods.EnableWindow(_hwnd, value); }
         }
 
-        public Size MaxWindowSize
+        public Size MaxClientSize
         {
             get
             {
                 return new Size(
                     UnmanagedMethods.GetSystemMetrics(UnmanagedMethods.SystemMetric.SM_CXMAXTRACK),
-                    UnmanagedMethods.GetSystemMetrics(UnmanagedMethods.SystemMetric.SM_CYMAXTRACK));
+                    UnmanagedMethods.GetSystemMetrics(UnmanagedMethods.SystemMetric.SM_CYMAXTRACK))
+                    - BorderThickness;
             }
         }