[BUG] Media Aside block only partly functional

[jjsanderson]

Describe the bug
(posted for tracking purposes, I suspect you're aware of this already)

The Media Aside block is flaky, in that the button to accept a link URL passes behind the image (or image placeholder), and hence can't be clicked.

Platform (please complete the following information):

• OS: OS X
• Tested in Safari and Chrome (current as of 2019-06-19)