QML WebView not using username and password parts of https_proxy?

[ChTh]

I'm on Linux and trying to get a WebView in QML to show an HTTPS-webpage via a proxy.

I have set "export https_proxy='https://USERNAME:PASSWORD@192.168.0.100:808'"

wget works fine with this setting, I can see it connect in the proxy server logs.

This small example does not though, when run with qmlscene:

import QtQuick 2.2
import QtQuick.Window 2.1
import QtWebKit 3.0

Window {
visible: true
width: 360
height: 360

WebView {
    id: webview
    anchors.fill: parent
    url: "https://accounts.google.com"
    //url: "http://www.yahoo.com"
}

}

All other cases seem to work, e.g. username and password for HTTP-proxy, accessing HTTPS without proxy, etc.

Does anyone know of anything inherent in QtWebKit that makes username and password in https_proxy fail?