I've had this happen a handful of times where I'm scrolling through posts and the video ad pops up in the bottom right and the vertical scroll bar disappears. I'm unable to scroll on the page after that without a refresh.
I've only experienced this on Firefox and I'm not sure if I'm alone but it's happened on multiple PCs.
I've only experienced this on Firefox and I'm not sure if I'm alone but it's happened on multiple PCs.