to reproduce, open two windows with scrollbar, make one unfocused, go in a proximity area of the unfocused window, make the thumb appear and click on it: the thumb wil disappear -> now make it appear again ad you'll scroll until you click on it (in few words, problems with grab...)
to reproduce, open two windows with scrollbar, make one unfocused, go in a proximity area of the unfocused window, make the thumb appear and click on it: the thumb wil disappear -> now make it appear again ad you'll scroll until you click on it (in few words, problems with grab...)