The same issue was fixed in content-on-the-fly-prototype bundle on EZP-27253, but it seems that the issue is not solved on Page edit.
This happens while using Firefox 52.0.2 (64bit).

Steps to reproduce

• create or edit a landing page
• drag an embed block into the page
• while adding an embed content, choose to create a new content
• choose the default "Article" content type
• choose a location
• the page with the New "Article" form is displayed
• scrolling on the form will be impossible

If the content type has many fields it will not be possible to reach them, because there's to way to scroll down.