Support Window transparency

src/Eto.Gtk/Forms/GtkWindow.cs

@@ -336,9 +336,12 @@ public bool MovableByWindowBackground
 				if (Widget.Properties.TrySet(GtkWindow.MovableByWindowBackground_Key, value))
 				{
 					if (value)
-						Control.ButtonPressEvent += Connector.ButtonPressEvent_Movable;
+					{
+						containerBox.AddEvents((int)Gdk.EventMask.ButtonPressMask);
+						containerBox.ButtonPressEvent += Connector.ButtonPressEvent_Movable;
+					}
 					else
-						Control.ButtonPressEvent -= Connector.ButtonPressEvent_Movable;
+						containerBox.ButtonPressEvent -= Connector.ButtonPressEvent_Movable;
 				}
 			}
 		}
@@ -547,11 +550,36 @@ internal void ButtonPressEvent_Movable(object o, Gtk.ButtonPressEventArgs args)
 				if (handler == null)
 					return;
 				var evt = args.Event;
-				if (handler != null && evt.Type == Gdk.EventType.ButtonPress && evt.Button == 1)
-				{
+				if (!Equals(args.RetVal, true) && evt.Type == Gdk.EventType.ButtonPress && evt.Button == 1)
+				{					
 					handler.Control.BeginMoveDrag((int)evt.Button, (int)evt.XRoot, (int)evt.YRoot, evt.Time);
+					args.RetVal = true;
+				}
+			}
+
+			[GLib.ConnectBefore]
+			internal void HandleDrawnEvent(object o, Gtk.DrawnArgs args)
+			{
+				var handler = Handler;
+				if (handler == null)
+					return;
+				// use cairo to draw the background color with alpha
+				var cr = args.Cr;
+				var color = handler.BackgroundColor;
+				cr.Save();
+				if (color.A > 0)
+				{
+					cr.SetSourceRGBA(color.R, color.G, color.B, color.A);
+					cr.Operator = Cairo.Operator.Source;
+				}
+				else
+				{
+					cr.Operator = Cairo.Operator.Clear;
 				}
+				cr.Paint();
+				cr.Restore();
 			}
+
 		}
 
 		public MenuBar Menu
@@ -925,5 +953,38 @@ public override void InvalidateMeasure()
 				Application.Instance.AsyncInvoke(PerformResize);
 			}
 		}
+
+		public override Gtk.Widget BackgroundControl => Control;
+
+		bool isDrawingBackground;
+
+		protected override void SetBackgroundColor(Color? color)
+		{
+			if (color?.A < 1 || isDrawingBackground)
+			{
+				if (!isDrawingBackground)
+				{
+					Control.AppPaintable = true;
+					var screen = Gdk.Screen.Default;
+					var visual = screen.RgbaVisual;
+
+					if (visual != null && screen.IsComposited)
+						Control.Visual = visual;
+
+					Control.Drawn -= Connector.HandleDrawnEvent;
+					Control.Drawn += Connector.HandleDrawnEvent;
+					isDrawingBackground = true;
+					base.SetBackgroundColor(null);
+				}
+
+				Control.QueueDraw();
+			}
+			else
+			{
+				Control.AppPaintable = false;
+				Control.Drawn -= Connector.HandleDrawnEvent;
+				base.SetBackgroundColor(color);
+			}
+		}
 	}
 }

src/Eto.Mac/Forms/MacWindow.cs

@@ -261,6 +261,11 @@ public bool MovableByWindowBackground
 
 		protected override Color DefaultBackgroundColor => NSColor.WindowBackground.ToEtoWithAppearance();
 
+		protected override void SetBackgroundColor(Color? color)
+		{
+			Control.BackgroundColor = (color ?? DefaultBackgroundColor).ToNSUI();
+		}
+
 		protected override SizeF GetNaturalSize(SizeF availableSize)
 		{
 			SizeF naturalSize = new SizeF(200, 200);

src/Eto.WinForms/Forms/WindowsControl.cs

@@ -662,13 +662,15 @@ public virtual Color BackgroundColor
 			set
 			{
 				backgroundColorSet = true;
+				var color = value.ToSD();
 				try
 				{
-					Control.BackColor = value.ToSD();
+					Control.BackColor = color;
 				}
 				catch
 				{
-					// some controls don't support transparent colors, ignore..
+					// some controls don't support transparent colors, set only RGB..
+					Control.BackColor = sd.Color.FromArgb(255, color);
 				}
 			}
 		}

src/Eto.WinForms/Win32.cs

@@ -130,8 +130,10 @@ public enum WS : uint
 		public enum WS_EX : uint
 		{
 			TOPMOST = 0x00000008,
+			TRANSPARENT = 0x00000020,
 			TOOLWINDOW = 0x00000080,
 			APPWINDOW = 0x00040000,
+			LAYERED = 0x00080000,
 			NOACTIVATE = 0x08000000
 		}
 

src/Eto.Wpf/Forms/WpfWindow.cs

@@ -1,6 +1,7 @@
 using Eto.Wpf.CustomControls;
 using Eto.Wpf.Forms.Menu;
 using Eto.Wpf.Drawing;
+using System.Windows.Input;
 
 namespace Eto.Wpf.Forms
 {
@@ -84,22 +85,23 @@ public bool MovableByWindowBackground
 			{
 				if (Widget.Properties.TrySet(WpfWindow.MovableByWindowBackground_Key, value))
 				{
-					if (value)
-						content.MouseLeftButtonDown += Content_MouseLeftButtonDown;
-					else
-						content.MouseLeftButtonDown -= Content_MouseLeftButtonDown;
+					HandleEvent(Eto.Forms.Control.MouseDownEvent);
 				}
 			}
 		}
 
-		void Content_MouseLeftButtonDown(object sender, swi.MouseButtonEventArgs e)
+		protected override void HandleMouseDown(object sender, MouseButtonEventArgs e)
 		{
-			// mouse could be captured by something else, so we release it here to ensure the DragMove works.
-			// we only get here if no control has handled the mouse down event.
-			swi.Mouse.Captured?.ReleaseMouseCapture();
+			base.HandleMouseDown(sender, e);
+			if (!e.Handled && MovableByWindowBackground && e.GetEtoButtons() == MouseButtons.Primary)
+			{
+				// mouse could be captured by something else, so we release it here to ensure the DragMove works.
+				// we only get here if no control has handled the mouse down event.
+				swi.Mouse.Captured?.ReleaseMouseCapture();
 
-			Control.DragMove();
-			e.Handled = true;
+				Control.DragMove();
+				e.Handled = true;
+			}
 		}
 
 		protected override void Initialize()
@@ -955,8 +957,15 @@ public WindowStyle WindowStyle
 			{
 				if (WindowStyle != value)
 				{
+					if (isSourceInitialized && Control.AllowsTransparency)
+					{
+						Trace.WriteLine("WARNING: Changing window style after window is shown is not supported on this platform");
+						return;
+					}
+
 					Control.WindowStyle = value.ToWpf();
 					SetWindowChrome();
+					SetWindowTransparency();
 				}
 			}
 		}
@@ -1043,8 +1052,24 @@ public void SendToBack()
 
 		public override Color BackgroundColor
 		{
-			get { return content.Background.ToEtoColor(); }
-			set { content.Background = value.ToWpfBrush(content.Background); }
+			get => (Control.Background ?? sw.SystemColors.ControlBrush).ToEtoColor();
+			set
+			{
+				Control.Background = value.ToWpfBrush();
+				SetWindowTransparency();
+			}
+		}
+
+		void SetWindowTransparency()
+		{
+			var isTransparent = Control.Background is swm.SolidColorBrush scb && scb.Color.A < 255 && WindowStyle == WindowStyle.None;
+			if (isSourceInitialized)
+			{
+				if (isTransparent != Control.AllowsTransparency)
+					Trace.WriteLine("WARNING: Changing transparency after window is shown is not supported on this platform");
+				return;
+			}
+			Control.AllowsTransparency = isTransparent;
 		}
 
 		public Screen Screen

src/Eto.Wpf/themes/controls/Window.xaml

@@ -6,7 +6,10 @@
     xmlns:ef="clr-namespace:Eto.Wpf.Forms"
     xmlns:s="clr-namespace:System;assembly=mscorlib">
 
-    <Style TargetType="ef:EtoWindowContent">
+    <Style TargetType="ef:EtoFormWindow">
+        <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
+    </Style>
+    <Style TargetType="ef:EtoWindow">
         <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
     </Style>