The Message reply box is not fixed

The Message reply box is not fixed on the page and instead appears at the bottom of the user's message list.
This is problematic if one of these message categories(Starred, Group, or Private) is expanded and there are quite a few messages as the reply box can be hidden underneath the message list

STEPS TO REPLICATE

1. Login as an administrator - it is not necessary to be an admin, but it's easier to set up the test
2. Under Site administration> Users >Browse list of users, select a user by clicking on their profile name
3. From the user profile page, select 'Message'
4. Send a message to the user
5. After message is sent, click the ellipses/three dots and select 'Star conversation'
6. Repeat steps 2-5 until at least 10 messages have been sent to different users and starred
(You may need more than 10 messages depending on screen size)
7. Click on your personal user profile in the top right-hand corner and select 'Messages'
8. You should now see a menu under the search field with the following messaging 'categories':
'Starred', 'Group' , and 'Private'
9. Expand the 'Starred' category and select one of the user's messages
Note: You should see the message box jump to the bottom of the list now with a scroll bar added
10. Send more messages to that user by adding text until the text is longer than the expanded 'Star' category list

Observe that a new scroll bar is added once the message becomes longer than the expanded 'category'.
If you collapse the 'Starred' category, you will see that the message box jumps back into view and the 'category' scroll bar disappears.

BSERVED BEHAVIOR:

Message reply box appears at the bottom the user's message list, requiring the user to scroll down past all of their message to see the reply box,

EXPECTED BEHAVIOR:

Message reply box should appear fixed below the individual message, not below the message list.

REPLICATION LOCATIONS

https://core2next-sandbox.mrooms.net/message/index.php

Here is a video of the problem: https://www.screencast.com/t/UsjtNIAtDuJ5