We use hamburger menu on the Branches page to trigger deployments via your Sync branch option.
Recently the text area for adding a message disappeared.
I have noticed that if I first select Merge then cancel that dialog and then use Sync branch the commit message text area is available.
This is a branch specific "workaround".
See the two attached screen captures of my Chrome 67.0.3396.99 on Win 10 displaying the two different sync dialogs.