This may not be related, but I think I should mention it. I can observe very similar behaviour with Plank docked to the left side of my screen, as seen here - https://drive.google.com/open?id=0B8qh4-wxxEZ7b3ZTZ0stb1VvdGM
So maybe not only xfce4-panel is affected by this bug, but all docked (GTK3) windows/objects?
This may not be related, but I think I should mention it. I can observe very similar behaviour with Plank docked to the left side of my screen, as seen here - https:/ /drive. google. com/open? id=0B8qh4- wxxEZ7b3ZTZ0stb 1VvdGM
So maybe not only xfce4-panel is affected by this bug, but all docked (GTK3) windows/objects?