I'm looking through the UpdateViewportPosition(Window *w) function in viewport.cpp in the hope of fixing #7005, and I can't seem to figure out what the bool update_overlay variable is supposed to represent. Presumably it's supposed to tell me whether the cargo flow legend might need to be redrawn, but it's somehow only set to true when the current scroll position is equal to the destination scroll position (well, why do we need to redraw the cargo flow legend if we didn't change the scroll position?). I've tried enabling and disabling smooth scrolling, as well as enabling and disabling the cargo flow legend, but no matter what I do, I can't make update_overlay become true when playing the game.

Can someone please tell me exactly what I have to do in the game to make update_overlay become true? If it makes things any easier, it appears to be added by rubidium in 2013.