When visiting some websites, including Google, YouTube, and Craigslist, content slowly moves down the page leaving a blank space, which grows over time. Disabling AdBlock stops the blank space from appearing, but an ad is visible instead.
This issue is caused by malware on your computer. We recommend following instructions provided by The Safe Mac in their Adware Removal Guide to solve this issue.
Some users have also reported that running a scan using the avast! antivirus is also effective.