have a guess a fullscreen on x11.

src/Avalonia.X11/X11Atoms.cs

@@ -156,6 +156,7 @@ namespace Avalonia.X11
         public readonly IntPtr _NET_SYSTEM_TRAY_OPCODE;
         public readonly IntPtr _NET_WM_STATE_MAXIMIZED_HORZ;
         public readonly IntPtr _NET_WM_STATE_MAXIMIZED_VERT;
+        public readonly IntPtr _NET_WM_STATE_FULLSCREEN;
         public readonly IntPtr _XEMBED;
         public readonly IntPtr _XEMBED_INFO;
         public readonly IntPtr _MOTIF_WM_HINTS;

src/Avalonia.X11/X11Window.cs

@@ -555,6 +555,11 @@ namespace Avalonia.X11
                     ChangeWMAtoms(true, _x11.Atoms._NET_WM_STATE_MAXIMIZED_VERT,
                         _x11.Atoms._NET_WM_STATE_MAXIMIZED_HORZ);
                 }
+                else if (value == WindowState.FullScreen)
+                {
+                    ChangeWMAtoms(false, _x11.Atoms._NET_WM_STATE_HIDDEN);
+                    ChangeWMAtoms(true, _x11.Atoms._NET_WM_STATE_FULLSCREEN);
+                }
                 else
                 {
                     ChangeWMAtoms(false, _x11.Atoms._NET_WM_STATE_HIDDEN);
@@ -585,6 +590,12 @@ namespace Avalonia.X11
                             break;
                         }
 
+                        if(pitems[c] == _x11.Atoms._NET_WM_STATE_FULLSCREEN)
+                        {
+                            state = WindowState.FullScreen;
+                            break;
+                        }
+
                         if (pitems[c] == _x11.Atoms._NET_WM_STATE_MAXIMIZED_HORZ ||
                             pitems[c] == _x11.Atoms._NET_WM_STATE_MAXIMIZED_VERT)
                         {