Looks like this never quite worked when the area you are filling was offset. The last item to be placed wasn’t getting all the remaining space, leaving some unused.
To see the bug in action set up some components with a deliberate gap at the top i.e. StretchableLayoutManager::layOutComponents with y>0
and with the overal height greater than those implied by the initial values given to StretchableLayoutManager::setItemLayout()