Events are queued when rendering blocked

With the transition to 5.2.1 (we unfortunately skipped 5.1 and 5.2, so can't say exactly when that happened), something changed in the QML engine that causes events to be queued if the app can't render. In our case it's Mir stopping the buffers because screen is off, but this can be emulated using Xvfb and sending a SIGSTOP to it.

Real-world issues we encountered on Ubuntu:

• can't change volume when screen is off, all input events are delivered to the shell after screen is turned on again;
• EndOfStream event isn't delivered to the music app, resulting in no playlist progression when screen is off, next song plays as soon as screen is back on;

The following piece of code continues to print current date when rendering stops under Qt 5.0, but stops after a second or two under Qt 5.2.

import QtQuick 2.0

Item {
    width: 300
    height: 200
    Text {
        id: text
        text: Date()
        anchors.centerIn: parent
    }

    Timer {
        running: true
        repeat: true
        onTriggered: {
            var date = Date();
            console.log(date);
            text.text = date;
        }
    }
}

Steps to reproduce (on Ubuntu Trusty):

# screen definition needed for Gallium to work
xvfb-run -s "-screen 0 1024x768x24" qmlscene printDate.qml

# in another terminal
pkill -SIGSTOP Xvfb

# observe that output from QML stops after a few seconds

# after output stopped
pkill -SIGCONT Xvfb

# observer that output from QML resumes

In the case above there is no "burst" of queued messages because of how Timer works, but that's a special case.