Behavior not working in inheritance sometimes - qt

Question

I found strange behavior when using Behavior. I want to animate the height of a Pane when I changed the Pane's height:
ModulePanelCard.qml:
import QtQuick 2.7
import QtQuick.Controls 2.1
import QtQuick.Controls.Material 2.1
Pane{
id: root
property bool isToggled: true
Material.elevation: 6
height: root.isToggled? 300: 100
Behavior on height {
NumberAnimation{duration: 300}
}
Button{
text: "click"
anchors{top: parent.top; right: parent.right; left: parent.left}
height: 40
onClicked: root.isToggled = !root.isToggled
}
}
main.qml:
import QtQuick.Controls 2.1
import QtQuick.Controls.Material 2.1
import QtQuick 2.6
ApplicationWindow {
id: window
width: 400
height: 400
visible: true
ModulePanelCard{
id: strategyCard
width: 300
anchors{top:parent.top; right: parent.right; left: parent.left}
}
}
OK. The animation shows up and everything goes fine! However when I want to do some inheritance, i.e. inherit from ModulePanelCard, strange things happen:
StrategyCard.qml
import QtQuick 2.7
ModulePanelCard{
}
main.qml:
import QtQuick.Controls 2.1
import QtQuick.Controls.Material 2.1
import QtQuick 2.6
ApplicationWindow {
id: window
width: 400
height: 400
visible: true
StrategyCard{
id: strategyCard
width: 300
anchors{top:parent.top; right: parent.right; left: parent.left}
}
}
If I put nothing into StrategyCard, everything is the same as previous. However, when I insert even one property or signal to StrategyCard, the animation of the height is just gone! The Behavior on the height won't work anymore!
StategyCard.qml:
import QtQuick 2.7
ModulePanelCard{
property string xxx: "hello"
}
However, if I put the Behavior statement into StrategyCard.qml, then the animation comes back!
New StrategyCard.qml:
import QtQuick 2.7
ModulePanelCard{
property string xxx: "hello"
Behavior on height {
NumberAnimation{duration: 300}
}
}
What happened? Anyone helps me? Thanks!
My development environment is: Windows 10 64bit, Qt 5.9.0 64bit for VS2015, Qt Creator 4.3.0