I think this is a regression from Qt 5.10 (which works fine). When the Button/ToolButton width/height are manually set then the hover background is too big (ToolButton) or the text is outside the button (Button):

ToolButton {
    text: "T"
    width: 50
    height: width
    //clip: true
}

Button {
    text: "T"
    width: 100
    height: width
    //clip: true
}