Describe the bug
When editing a reusable block from a page or post, adding new blocks is impossible because the menu that pops up quickly closes and focus shifts back to the reusable block title.

To Reproduce
Steps to reproduce the behavior:

1. Create reusable block
2. Insert reusable block on page or post
3. Click the edit button next to reusable block's name.
4. Attempt to add a block inside the reusable block using the plus button.

Expected behavior
The menu should stay open until a block is selected.

Screenshots
Here is a screencast showing the issue: https://www.dropbox.com/s/7o33s00o6k1qlrc/gutenberg-reusable-issue.mp4?dl=0

Desktop (please complete the following information):
Linux Mint 18.3
Firefox 63, also happens in Chrome.

Smartphone (please complete the following information):
Not tested

Additional context
Gutenberg 4.6.1
WP 4.9.8 vanilla install 2017 theme.