Set WM_CLASS property according to ICCCM spec

src/Avalonia.X11/X11Platform.cs

@@ -277,7 +277,8 @@ namespace Avalonia
             // and sometimes attempts to use GLX might cause a segfault
             "llvmpipe"
         };
-        public string WmClass { get; set; } = Assembly.GetEntryAssembly()?.GetName()?.Name ?? "AvaloniaApplication";
+
+        public string WmClass { get; set; } = Assembly.GetEntryAssembly()?.GetName()?.Name;
 
         /// <summary>
         /// Enables multitouch support. The default value is true.

src/Avalonia.X11/X11Window.cs

@@ -168,8 +168,7 @@ namespace Avalonia.X11
             XChangeProperty(_x11.Display, _handle, _x11.Atoms._NET_WM_WINDOW_TYPE, _x11.Atoms.XA_ATOM,
                 32, PropertyMode.Replace, new[] {_x11.Atoms._NET_WM_WINDOW_TYPE_NORMAL}, 1);
 
-            if (platform.Options.WmClass != null)
-                SetWmClass(platform.Options.WmClass);
+            SetWmClass(_platform.Options.WmClass);
 
             var surfaces = new List<object>
             {
@@ -1082,12 +1081,24 @@ namespace Avalonia.X11
 
         public void SetWmClass(string wmClass)
         {
-            var data = Encoding.ASCII.GetBytes(wmClass);
-            fixed (void* pdata = data)
+            // See https://tronche.com/gui/x/icccm/sec-4.html#WM_CLASS
+            // We don't actually parse the application's command line, so we only use RESOURCE_NAME and argv[0]
+            var appId = Environment.GetEnvironmentVariable("RESOURCE_NAME") 
+                        ?? Process.GetCurrentProcess().ProcessName;
+            
+            var encodedAppId = Encoding.ASCII.GetBytes(appId);
+            var encodedWmClass = Encoding.ASCII.GetBytes(wmClass ?? appId);
+
+            var hint = XAllocClassHint();
+            fixed(byte* pAppId = encodedAppId)
+            fixed (byte* pWmClass = encodedWmClass)
             {
-                XChangeProperty(_x11.Display, _handle, _x11.Atoms.XA_WM_CLASS, _x11.Atoms.XA_STRING, 8,
-                    PropertyMode.Replace, pdata, data.Length);
+                hint->res_name = pAppId;
+                hint->res_class = pWmClass;
+                XSetClassHint(_x11.Display, _handle, hint);
             }
+
+            XFree(hint);
         }
 
         public void SetMinMaxSize(Size minSize, Size maxSize)

src/Avalonia.X11/XLib.cs

@@ -109,6 +109,9 @@ namespace Avalonia.X11
 
         [DllImport(libX11)]
         public static extern int XFree(IntPtr data);
+        
+        [DllImport(libX11)]
+        public static extern int XFree(void* data);
 
         [DllImport(libX11)]
         public static extern int XRaiseWindow(IntPtr display, IntPtr window);
@@ -628,6 +631,12 @@ namespace Avalonia.X11
             return XISelectEvents(display, window, emasks, devices.Count);
 
         }
+        
+        [DllImport(libX11)]
+        public static extern XClassHint* XAllocClassHint();
+
+        [DllImport(libX11)]
+        public static extern int XSetClassHint(IntPtr display, IntPtr window, XClassHint* class_hints);
 
         public struct XGeometry
         {
@@ -639,6 +648,11 @@ namespace Avalonia.X11
             public int bw;
             public int d;
         }
+        public struct XClassHint
+        {
+            public byte* res_name;
+            public byte* res_class;
+        }
         
         public struct XSyncValue {
             public int Hi;