If the font.family assignment is used on Windows then the Menu texts are written with elision, e. g. "Sub…". Showing width, implicitWidth, and contentWidth of the MenuBar stays the same, also the font looks the same with and without the assignment. With the assignment just the written text is truncated and elided, which is bad for a menu.

import QtQuick 2.9
import QtQuick.Controls 2.13
import QtQuick.Layouts 1.12

ApplicationWindow {
 id: root
 visible: true
 title: qsTr("Demo")
 menuBar: MenuBar {
  font.family: Qt.platform.os != "windows" ? "Liberation Sans" : Qt.application.font
  Menu
  { title: qsTr("Submenu") }
 }
}