Issue title: Fullscreen in Hero Wars Facebook not working
 Issue body: **Version info**:https://paste.the-compiler.org/view/082f87d9
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**Does the bug happen if you start with `--temp-basedir`?**:

**Description**

**How to reproduce**
https://apps.facebook.com/mobaheroes
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 Comments: 
 Comment 0: This is unlikely to be an issue specific to qutebrowser. Can you please try with another QtWebEngine-based browser such as Falkon, as well as with Chromium?
 Comment 1: Chromium: works
Falkon: same, not working
![](https://user-images.githubusercontent.com/17645894/103472176-5b43d400-4dc5-11eb-86ee-672c791ac879.png)

 Comment 2: Closing this as an upstream issue in [QtWebEngine](https://wiki.qt.io/QtWebEngine).

I try to report bugs upstream when I can reproduce them, but unfortunately this isn't the case here as I don't have a Facebook account. Since I'll be unable to provide any follow-up information when being asked, could you please report the bug to the [Qt Bugtracker](https://bugreports.qt.io/secure/Dashboard.jspa) yourself? Please use "Project: Qt (QTBUG)"  and "Component: WebEngine" there. After reporting the bug, it'd be good if you could post the link to it here!
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