I've noticed that on several occasions over the last day, when I open a new editing box, the buttons on the top row are missing. They aren't invisible, just gone. It also seems to disable the Ctrl+I and Ctrl+B shortcut keys.
This seems to be replicable if you open the editing box, then cancel/save it, then open it again. It can be fixed by reloading.
FYI: Chrome Version 53.0.2785.116 - Windows 8.1 Pro.