update documentation and fix bug when

system decorations are restored so that CanResize is obeyed.

src/Avalonia.Controls/Window.cs

@@ -214,7 +214,9 @@ namespace Avalonia.Controls
         }
 
         /// <summary>
-        /// Enables or disables resizing of the window
+        /// Enables or disables resizing of the window.
+        /// Note that if <see cref="HasSystemDecorations"/> is set to False then this property
+        /// has no effect and should be treated as a recommendation for the user setting HasSystemDecorations.
         /// </summary>
         public bool CanResize
         {

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -287,6 +287,21 @@ namespace Avalonia.Win32
                 UnmanagedMethods.SetWindowPosFlags.SWP_NOZORDER | UnmanagedMethods.SetWindowPosFlags.SWP_NOACTIVATE);
 
             _decorated = value;
+
+            if(_decorated)
+            {
+                if (_resizable)
+                {
+                    // If we switch decorations back on we need to restore WS_SizeFrame.
+                    _resizable = false;
+                    CanResize(true);
+                }
+                else
+                {
+                    _resizable = true;
+                    CanResize(false);
+                }
+            }
         }
 
         public void Invalidate(Rect rect)
@@ -857,14 +872,18 @@ namespace Avalonia.Win32
                 return;
             }
 
-            var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE);
-            
-            if (value)
-                style |= UnmanagedMethods.WindowStyles.WS_SIZEFRAME;
-            else
-                style &= ~(UnmanagedMethods.WindowStyles.WS_SIZEFRAME);
-            
-            UnmanagedMethods.SetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE, (uint)style);
+            if (_decorated)
+            {
+                var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE);
+
+                if (value)
+                    style |= UnmanagedMethods.WindowStyles.WS_SIZEFRAME;
+                else
+                    style &= ~(UnmanagedMethods.WindowStyles.WS_SIZEFRAME);
+
+                UnmanagedMethods.SetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE, (uint)style);
+            }
+
             _resizable = value;
         }
     }