WebEngineView (QML) does not pass mouse press/release to webpage

I run this html file (Test2.html) inside a WebEngineView. However the mouse up and mouse down events do not pass to the webpage. The mouse click is OK thought.

I tried the html file both in Google Chrome and IE 11, and they both work fine. Someone ask me to try out QupZilla because it uses WebEngineView. QupZilla do the same as my own test app...not work.

(My test app is just a simple QML app with a WebEngineView set url to the Test.html file.)