I reported my first bug with WordPress today. During testing of my WordPress plugins for WP 4.0 compatibility, Admin Classic Borders worked perfectly but I found an odd occurrence with Featured Galleries. Using it seemed to break the post editing screen.
Further testing proved Featured Galleries innocent though, because I was able to replicate the issue with all plugins disabled. Turns out that a new featured of WP 4.0 is a sticky header, which follows you as you scroll down when typing posts. It works by using JS to detect when a user starts to scroll, and changing the header from normal positioning to fixed, which lets it follow you. When a user scrolls upwards, the fixed positioning is eventually removed.
The problem occurs when you are scrolled down enough to cause the fixed positioning to kick in, and then suddenly are at the top of the page, without scrolling up. The can occur when remove a metabox, such as featured image. This causes the page to shrink suddenly, but doesn’t trigger the JS to fix the header.
Hopefully I found it early enough to be fixed before 4.0 comes out. You can follow along, and read more about it, here:
EDIT: Ticket was just assigned to the 4.0 milestone, meaning it has to be fixed before 4.0 can be released.
EDIT 2: Bug has been fixed.