reapply window.open() clamping

In alpha we seem to have lost the 10px clamping for new windows (which are opened in a new tab: see #9881 (moved) )

• See https://bugzilla.mozilla.org/show_bug.cgi?id=1556016
• See https://ghacksuserjs.github.io/TorZillaPrint/TorZillaPrint.html - click the new window button

Pic to follow: example of a user resizing the browser for more real estate: which we want to encourage for uptake and for the letterboxing buckets to be less FP'able (I assume).

Would be good to upstream it