Windows open outside the visible part of the site. For example, if the visual editor click on the “Link” or “Video”, a new window will open far below, under the footer.


It could be a couple of things. Can you inspect it using the browsers console / inspector?
It’s possible something from the WP theme is taking over the CSS and pushing the window down.
Alternatively if you are running any TinyMCE plugins, even if only for use in the back end they could still affect the editor here on the forum. If you are it might be worth disabling and seeing if it resolves the problem.