Steps to reproduce the issue

Install Joomal 4.0-dev @ 1f24603
Login to super admin
Click Install next to Blog Sample Data
Click ok in JS popup

Go to front end and

Login as super admin

click Welcome to your blog
Click cog -> click edit (to edit the article)

Click "CMS Content" button in editor
Select Page Break
Enter a title
Enter a Table of contents alias
Click Insert page break

Expected result

No console errors
pagebreak inserted to wysiwyg

Actual result

TypeError: null is not an object (evaluating 'r.pageYOffset')

Modal loads frontend homepage

System information (as much as possible)

safari on Mac

Additional comments