[fix] position windows before showing to prevent flicker

apps/desktop/src/gui/application/window_presentation.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <slint.h>
+
+namespace crossdesk {
+
+// Winit creates hidden native windows on a later event-loop turn. Wait for
+// that window before applying geometry that depends on its frame or DPI.
+inline bool NativeWindowReady([[maybe_unused]] const slint::Window& window) {
+#if defined(_WIN32)
+  return window.win32_hwnd() != nullptr;
+#elif defined(__APPLE__)
+  return window.appkit_view() != nullptr;
+#else
+  // Other backends accept initial geometry before show(); Wayland placement
+  // is controlled by the compositor and must not wait for a mapped surface.
+  return true;
+#endif
+}
+
+}  // namespace crossdesk