Implement topmost on window

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -55,6 +55,11 @@ namespace Avalonia.Platform
         /// </summary>
         void CanResize(bool value);
 
+        /// <summary>
+        /// Gets or sets whether this window appears on top of all other windows
+        /// </summary>
+        void SetTopmost(bool value);
+
         /// <summary>
         /// Gets or sets a method called before the underlying implementation is destroyed.
         /// Return true to prevent the underlying implementation from closing.

src/Avalonia.Controls/Window.cs

@@ -104,6 +104,9 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<bool> CanResizeProperty =
             AvaloniaProperty.Register<Window, bool>(nameof(CanResize), true);
 
+        public static readonly StyledProperty<bool> TopmostProperty =
+            AvaloniaProperty.Register<Window, bool>(nameof(Topmost));
+
         private readonly NameScope _nameScope = new NameScope();
         private object _dialogResult;
         private readonly Size _maxPlatformClientSize;
@@ -125,6 +128,8 @@ namespace Avalonia.Controls
 
             CanResizeProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.CanResize((bool)e.NewValue));
 
+            TopmostProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetTopmost((bool)e.NewValue));
+
             WindowStateProperty.Changed.AddClassHandler<Window>(
                 (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.WindowState = (WindowState)e.NewValue; });
         }
@@ -230,6 +235,15 @@ namespace Avalonia.Controls
             set { SetValue(CanResizeProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets whether this window appears on top of all other windows
+        /// </summary>
+        public bool Topmost
+        {
+            get { return GetValue(TopmostProperty); }
+            set { SetValue(TopmostProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets the icon of the window.
         /// </summary>

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -102,5 +102,9 @@ namespace Avalonia.DesignerSupport.Remote
         public void CanResize(bool value)
         {
         }
+
+        public void SetTopmost(bool value)
+        {
+        }
     }
 }

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -104,6 +104,10 @@ namespace Avalonia.DesignerSupport.Remote
         public void CanResize(bool value)
         {
         }
+
+        public void SetTopmost(bool value)
+        {
+        }
     }
 
     class ClipboardStub : IClipboard

src/Gtk/Avalonia.Gtk3/Interop/Native.cs

@@ -261,10 +261,13 @@ namespace Avalonia.Gtk3.Interop
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate void gtk_window_unmaximize(GtkWindow window);
-            
+
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate void gtk_window_close(GtkWindow window);
 
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
+            public delegate void gtk_window_set_keep_above(GtkWindow gtkWindow, bool setting);
+
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate void gtk_window_set_geometry_hints(GtkWindow window, IntPtr geometry_widget, ref GdkGeometry geometry, GdkWindowHints geom_mask);
 
@@ -472,6 +475,7 @@ namespace Avalonia.Gtk3.Interop
         public static D.gtk_window_maximize GtkWindowMaximize;
         public static D.gtk_window_unmaximize GtkWindowUnmaximize;
         public static D.gtk_window_close GtkWindowClose;
+        public static D.gtk_window_set_keep_above GtkWindowSetKeepAbove;
         public static D.gdk_window_begin_move_drag GdkWindowBeginMoveDrag;
         public static D.gdk_window_begin_resize_drag GdkWindowBeginResizeDrag;
         public static D.gdk_event_request_motions GdkEventRequestMotions;

src/Gtk/Avalonia.Gtk3/WindowImpl.cs

@@ -81,7 +81,9 @@ namespace Avalonia.Gtk3
         public void ShowTaskbarIcon(bool value) => Native.GtkWindowSetSkipTaskbarHint(GtkWidget, !value);
 
         public void CanResize(bool value) => Native.GtkWindowSetResizable(GtkWidget, value);
-        
+
+        public void SetTopmost(bool value) => Native.GtkWindowSetKeepAbove(GtkWidget, value);
+
 
         class EmptyDisposable : IDisposable
         {

src/OSX/Avalonia.MonoMac/WindowImpl.cs

@@ -116,6 +116,11 @@ namespace Avalonia.MonoMac
             UpdateStyle();
         }
 
+        public void SetTopmost(bool value)
+        {
+            Window.Level = value ? NSWindowLevel.Floating : NSWindowLevel.Normal;
+        }
+
         public void SetTitle(string title) => Window.Title = title;
 
         class ModalDisposable : IDisposable

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -78,6 +78,14 @@ namespace Avalonia.Win32.Interop
             SWP_RESIZE = SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOZORDER
         }
 
+        public static class WindowPosZOrder
+        {
+            public static readonly IntPtr HWND_BOTTOM = new IntPtr(1);
+            public static readonly IntPtr HWND_TOP = new IntPtr(0);
+            public static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);
+            public static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);
+        }
+
         public enum SizeCommand
         {
             Restored,

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -32,6 +32,7 @@ namespace Avalonia.Win32
         private bool _trackingMouse;
         private bool _decorated = true;
         private bool _resizable = true;
+        private bool _topmost = false;
         private double _scaling = 1;
         private WindowState _showWindowState;
         private WindowState _lastWindowState;
@@ -901,5 +902,21 @@ namespace Avalonia.Win32
 
             _resizable = value;
         }
+
+        public void SetTopmost(bool value)
+        {
+            if (value == _topmost)
+            {
+                return;
+            }
+
+            IntPtr hWndInsertAfter = value ? WindowPosZOrder.HWND_TOPMOST : WindowPosZOrder.HWND_NOTOPMOST;
+            UnmanagedMethods.SetWindowPos(_hwnd,
+                   hWndInsertAfter,
+                   0, 0, 0, 0,
+                   SetWindowPosFlags.SWP_NOMOVE | SetWindowPosFlags.SWP_NOSIZE | SetWindowPosFlags.SWP_NOACTIVATE);
+
+            _topmost = value;
+        }
     }
 }