I had the same issue has katkhal. It’s attributed to the responsiveness of the page. If you drop below a certain threshold the buttons in the WYSIWYG editor will stack causing the first two lines (and more on smaller browsers) of the editor to be hidden. This is more annoying when you’re starting a new page because it seems like it’s broken when you can’t access the first line to start the page.
I hope this helps someone else out. I thought I was going to have to reinstall WordPress there for a minute.
For reference I’m using WordPress 4, Chrome (latest), Windows 7.
