Busy.org: comment collapsed button is a little bit truncated on top comment which has sub comments

issue

In chrome desktop version, when a user move their mouse on the comment, the comment collapsed button is shown up to collapse comment. But when the comment has sub comments, the root comment's collapsed button is located on incorrect position which is too close to the top . With its wrong position, it is placed on underneath the another UI and it’s not a circle shape which is the normal button UI.

expected result

In desktop, when a user move their mouse on the comment, the comment collapsed button should be shown up on its position.
correct position

testing result

some comment which has some sub comments has incorrect comment collapsed button position.

wrong position

test evironment

Chrome browser version 63.0.3239.132 on Mac Sierra

H2
H3
H4
3 columns
2 columns
1 column
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