Somehow, this binding got removed without addressing the topic editing functionality: https://dxr.mozilla.org/comm-esr60/source/mail/components/im/content/chat.css#81 in TB 68. Both the issues, this and Bug 1559641 are because of this removal. I will submit the patch for Bug 1559641 after receiving feedback from this patch.
Bug 1523419 Comment 8 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Somehow, this binding got removed without addressing the topic editing functionality: https://dxr.mozilla.org/comm-esr60/source/mail/components/im/content/chat.css#81 in TB 68. Both the issues, this and Bug 1559641 are because of this removal. I will submit the patch for Bug 1559641 after receiving the feedback from this patch.