1. In FF 53 login in to a wiki with Flow enabled and switch to the monobook skin.
2. On a flow topic click on the empty star icon to add it to your watchlist or find a topic that is already added to your watchlist.
3. When the star is black, hover over it with the cursor - the star icons twinkles, There is a position of the cursor when the star icon looks as completely disappeared.

The "disappearance" effect is very subtle and transitory.