When we open a background tab, it gets a hardcoded size (800x600 or so) because
it doesn't get resized by the layout yet.
By resizing it to the size it'll actually have later, we make sure scrolling to
an anchor in an background tab works, and JS also gets the correct size for
background tabs.
Fixes#1190Fixes#2495
See #1417
The test for QtWebEngine was removed because something weird happened
with the two pytest-bdd tags, and I'm too annoyed with everything being
broken to investigate now... Future me, I'm sorry.
