Windows XP/ANGLE: Qt5.1 has redraw problem when obscured

This only occurs on Windows XP, using 5.1.0 alpha (ANGLE version).
5.0.2(ANGLE) and 5.1.0alpha(opengl) DON'T have this issue. It seemed be a regression compared with 5.0.2(ANGLE)

import QtQuick 2.0

Rectangle {
	width: 400; height: 300
	property bool flag: false
	color: flag ? "green" : "red"
	MouseArea {
		anchors.fill: parent
		onClicked: {
			parent.flag = ! parent.flag;
		}
	}
}

How to reproduce:
0) On Windows >= Windows XP, switch theme to "Windows Classic"
1).Start the program, a red window (called Qtwindow) occurs. It turns green when clicked. When clicked again it will turns to red again.
2).Move another window (called window2) above the Qtwindow, overlapping a part of that window.
3).Click Qtwindow [becomes active top window], then only the overlapped part of the window is redrawed (i.e. turns green or red)