Why user is able to edit comments in multiple textareas in the same time?

Question

By accident I have clicked edit link on comment twice.

After first click I have textarea for editing it (it is normal and expected behavior).

But on second and third click on edit I get garbage:

• second

  

• third

  

Seems to be an unexpected behavior or bug.
If I remember correctly, it did not exist in previous version of site layout.

I'm using Firefox ESR 52.9.0 if it matters.

Can somebody confirm this issue?