[X11] Emulate synchronous resize/move events until window is mapped for the first time

src/Avalonia.X11/X11Window.cs

@@ -41,6 +41,7 @@ namespace Avalonia.X11
         private IntPtr _xic;
         private IntPtr _renderHandle;
         private bool _mapped;
+        private bool _wasMappedAtLeastOnce = false;
         private HashSet<X11Window> _transientChildren = new HashSet<X11Window>();
         private X11Window _transientParent;
         private double? _scalingOverride;
@@ -769,6 +770,7 @@ namespace Avalonia.X11
         
         void ShowCore()
         {
+            _wasMappedAtLeastOnce = true;
             XMapWindow(_x11.Display, _handle);
             XFlush(_x11.Display);
         }
@@ -816,7 +818,7 @@ namespace Avalonia.X11
                 XConfigureResizeWindow(_x11.Display, _renderHandle, pixelSize);
             XFlush(_x11.Display);
 
-            if (force || (_popup && needImmediatePopupResize))
+            if (force || !_wasMappedAtLeastOnce || (_popup && needImmediatePopupResize))
             {
                 _realSize = pixelSize;
                 Resized?.Invoke(ClientSize);
@@ -857,6 +859,11 @@ namespace Avalonia.X11
                 XConfigureWindow(_x11.Display, _handle, ChangeWindowFlags.CWX | ChangeWindowFlags.CWY,
                     ref changes);
                 XFlush(_x11.Display);
+                if (!_wasMappedAtLeastOnce)
+                {
+                    _position = value;
+                    PositionChanged?.Invoke(value);
+                }
 
             }
         }