Describe the bug
The triangle that connects the block inserter selection to the "Add Block +" button is disconnected from the block.

To Reproduce
Steps to reproduce the behavior:

1. Go to "Add New Post"
2. Click on "Add block (+)" on the left of the empty block
3. Observe the disconnect!

Expected behavior
It's connected.

Screenshots

Additional context
Tested in Chrome and Firefox on master; probably happens elsewhere too.