Focus detection bug fix (case #29)

Fix wrong focus detection in case another window overlaps the browser window.
Link to the expected focus detection: https://docs.google.com/a/doubleverify.com/spreadsheets/d/1542E5-3MompVig-kQPuF2xtXSuVKGYmGB5HrN8_qa8U/edit#gid=0