You got me there! I’m not sure what the cause it; it wasn’t always reproducible for me, but I did see it a few times as well.
My best guess would be to try changing the Content Overflow setting in the Metrics inspector from Scrollbars to Visible. As it is an iframe it will display its own scrollers when needed. I couldn’t reproduce it with this setting, but that may have just been coincidence; it is the only thing that sticks out for me.