Describe the bug
On a site with a number of legacy metaboxes, if I scroll up a fair amount, I can never scroll down far enough to reach the last block, in effect, making it impossible to continue writing.

To Reproduce
Steps to reproduce the behavior:

1. Install a number of plugins that create legacy metaboxes: Jetpack, Genesis theme, Liveblog, Syndication Links, etc
2. Write a number of blocks to ensure plenty of scrolling is needed.
3. Scroll to the top.
4. Scroll to the bottom.

Expected behavior
Ability to scroll to the last block.
What happens: The scroll ability stops before the last block comes into view.

Screenshots

Desktop (please complete the following information):

• OS: Mac OS X latest
• Browser: Chrome
• Version 70.0.3538.110

Additional context
On WordPress svn branches/5.0 with RC3 plus anything committed as of r43798-build.