Use window innerWidth/Height instead of documentElement bounding box

[antonok-edm]

apparently, document.documentElement.getBoundingClientRect can have a height of 0 despite having a sensible clientHeight. However, the clientHeight property has its own quirks. window.innerHeight may be the best option available.

observed on www.france24.com (although not added as a testcase yet since there is a false positive for the keyword check)