The box appears as though it could be due to some browser cached item; or
maybe something in Safari. If you force quit Safari and then restart it with the
extensions-off (hold shift key on launch of Safari) that may help.
I've not had that kind of issue; others have and the source(s) may relate to
adware or some thing related to popups, and accidentally downloading
unwanted or unintended software, or just extensions in the browser itself.
The following support link covers several possibilities to your stated issue:
• Stop pop-up ads and adware in Safari - Apple Support
You may need to look further for other kinds of possibly related bits. And
never expect a new OS X to fix an underlying problem that predated it.
Sometimes the cause will be moved along with your old user account.
You could try to see what Console logs suggest, or look at the highlights
in an etrecheck report. Not sure if something such as adwaremedic helps.
Regarding the original topic of this thread, this of course -- isn't about that. LOL 🙂