My private tabs close themselves automatically, and I haven't been able to make it stop. I don't mean when I turn off/restart my phone, or when I close the app itself, but when I simply leave the app for a while. It seems to be time-based, but without much of a pattern to it. So basically I have private tabs open, hit the home button and do something else for ten or more minutes, and come back to find the tabs gone. How can I stop this from happening?