As summary (and as alluded to in QTBUG-120138). Any Quick Window instances have blank title bars under WebAssembly. This is because QWasmWindow::setWindowFlags sets the relevant CSS properties ultimately based on the contents of the Window.flags QML property, by default simply Qt.Window. If the flags property is manually set to include Qt.WindowTitleHint and Qt.WindowCloseButtonHint etc., the title bar visual elements appear as expected.

It's not intuitive to have to manually specify these things and indeed doesn't match the behaviour of other platforms so something is clearly wrong here. Browsing the source I wonder if QWasmWindow needs to include something like fixTopLevelWindowFlags from qwindowswindow.cpp? Not sure.