It's a "feature" to prevent battery draining. Current Opera releases also suspend updating the screen when in background. It's not an OS thing, as the OS merely reports to apps when they're being pushed to the background. Apps can then do as they see fit - and that's why iOS and Windows Phone sucks.

QtQuick has a WebView element. Perhaps that one continues to run in the background. However, other than creating an app out of it I've no idea how to test this quickly. (Is there a default QML viewer somewhere around? Or perhaps Python?)