WM_PAINT must always be handled.

Timers won't be called if not as WM_PAINT has a higer priority.

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -537,18 +537,15 @@ namespace Avalonia.Win32
                     break;
 
                 case UnmanagedMethods.WindowsMessage.WM_PAINT:
-                    if (Paint != null)
-                    {
-                        UnmanagedMethods.PAINTSTRUCT ps;
+                    UnmanagedMethods.PAINTSTRUCT ps;
 
-                        if (UnmanagedMethods.BeginPaint(_hwnd, out ps) != IntPtr.Zero)
-                        {
-                            UnmanagedMethods.RECT r;
-                            UnmanagedMethods.GetUpdateRect(_hwnd, out r, false);
-                            var f = Scaling;
-                            Paint(new Rect(r.left / f, r.top / f, (r.right - r.left) / f, (r.bottom - r.top) / f));
-                            UnmanagedMethods.EndPaint(_hwnd, ref ps);
-                        }
+                    if (UnmanagedMethods.BeginPaint(_hwnd, out ps) != IntPtr.Zero)
+                    {
+                        UnmanagedMethods.RECT r;
+                        UnmanagedMethods.GetUpdateRect(_hwnd, out r, false);
+                        var f = Scaling;
+                        Paint?.Invoke(new Rect(r.left / f, r.top / f, (r.right - r.left) / f, (r.bottom - r.top) / f));
+                        UnmanagedMethods.EndPaint(_hwnd, ref ps);
                     }
 
                     return IntPtr.Zero;