I found this topic Messenger sidebar doesn't show anymore, but no one has answerd that. I have the same problem. It just flashes and then dissapear. Some times it stays if I click like 47 times, but not always. I have tryed different versions, developer version, and I have had this problem for a long time.
Running: Ubuntu 16.04.3 LTS
Opera version right now: 51.0.2830.26
Does anyone have any suggestion how to solve the problem?