Glitchy editing experience when duplicating XBlock children

[kdmccormick]

Scenario

As a course author viewing the children of a Container block in the new React-based Unit editor, I want to duplicate an existing child block.

Actual

The other children disappear, and the duplicated block's editor is constrained to a tiny window. It does successfully duplicate the block, though.

Expected

Any of these:

• Editor should pop up over existing children
• Editor should be inline with existing children
• Duplicate should just pop in line with existing children, and author can then choose to edit it