Using the linked application, you should see:

The app is nicely placed on the browser. The MainWindow bars are at the correct place. The QQuickView inside fills the inside. Everything is fine

Using the linked sidebyside.html instead, we have this behavior:

The QMainWindow still follows correctly the "screen" html node. But the QQuickView instead does not.

We actually see on the devtool that it managed to calculate itself the offset:

The natural approach would be to fit the QMainWindow content, so having left: 0;

Maybe related to: https://bugreports.qt.io/browse/QTBUG-115054