According to comments related to the investigation, the interface repeatedly checks whether all buttons fit within the available horizontal space. If the controls overflow, the system hides one of the buttons to free space. However, hiding the button changes the container’s width, immediately creating a new problem.

Once the button disappears, the available width appears enough for the interface to believe there is room again, causing the hidden button to reappear. The buttons then overflow once more, forcing the interface to hide the button again. The cycle repeats continuously at extremely high speeds.

I wonder if this affects any pages that have YouTube videos embedded as well.