"X UNREAD" banner/badge gets stuck

The X UNREAD banner/badge/notification button remains no matter how many times it is clicked.

This happens occasionally starting about one or two months ago (October or November 2019).

Previously discussed in https://gitlab.com/gitlab-org/gitter/webapp/issues/2365 but created a new issue per the recommendation of @MadLittleMods

Reports

• @zipswich (me)
• Multiple people of Gitter room https://gitter.im/uno-platform/Lobby have reported this issue.

Reproduction instructions

There are simpler and more ways to reproduce this in the MR to fix this problem: https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1871#ways-to-reproduce-the-bug

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As a pre-requisite, enable threaded conversations for the room.

1. Send a couple messages in the main message feed
2. Start a thread on the last message
3. Send another message
4. Now the other user will have a stuck "X unread" banner. Clicking it does nothing
5. Refreshing will show the thread parent message as unread

https://gitlab.com/gitlab-org/gitter/webapp/issues/2388#note_283381594