QHighDPI: Fix wrong conversion for native child windows

Do not try to find a screen for native child coordinates.

Fixes: QTBUG-78158
Change-Id: I78ba814929f4db3dfd7dd43c09f7c7642222f4fb
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/gui/kernel/qhighdpiscaling.cpp

@@ -476,8 +476,10 @@ QHighDpiScaling::ScaleAndOrigin QHighDpiScaling::scaleAndOrigin(const QWindow *w
 {
     if (!m_active)
         return { qreal(1), QPoint() };
+
     QScreen *screen = window ? window->screen() : QGuiApplication::primaryScreen();
-    return scaleAndOrigin(screen, nativePosition);
+    const bool searchScreen = !window || window->isTopLevel();
+    return scaleAndOrigin(screen, searchScreen ? nativePosition : nullptr);
 }
 
 #endif //QT_NO_HIGHDPISCALING