Hello! I found the bug on Opera Stable 51. When I changed active display from fullscreen Opera to Desctop, and to fullscreen Opera again by trackpad gesture of macOS, the strange bar showed on the top of tabbar. Like this video↓
Incidentally, I sent bug report. And I use Opera Stable 51 (51.0.2830.26) on macOS High Sierra 10.13.3.
I forgot to add images of this bug. So, I added these images.