Question: Safari new popup window problem
For a while, when I go to a new web page in Safari on my iMac, it happens very often that the content on that web page opens up in a SECOND browser window on top of the ordinary web page. This new window always has a white background and displays only text and simple images from what looks like the main article on that page. If I click besides this browser window (on the ordinary web page in the background), the window disappears and I can scroll on the total page.
I can't figure out how to get rid of this - feature or bug or whatever. Any clue?
iMac (Retina 5K, 27-inch, Late 2014), macOS Sierra (10.12.6), iPad 3, iPhone5