Material style hover effects inconsistent between similarly-sized ToolButtons

import QtQuick 2.7
import QtQuick.Layouts 1.1
import QtQuick.Controls 2.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    Shortcut {
        sequence: "Ctrl+Q"
        onActivated: Qt.quit()
    }

    header: ToolBar {
        RowLayout {
            ToolButton {
                text: qsTr("File")
                hoverEnabled: true
            }

            ToolButton {
                text: qsTr("Edit")
                hoverEnabled: true
            }

            ToolButton {
                text: qsTr("Tools")
                hoverEnabled: true
            }
        }
    }
}