Default window's scroll bar thumb track draws thumb in old position

As indicated by Notepad's vertical scroll bar behavior when tracking thumb with text files that exceed window size, the thumb is drawn at old position when mouse moves, and at new position when it's not, which makes it flicker. This seems to occur in windows that use default window scrolling.

it seems that in user32/controls/scrollbar.c:947, new information gets set but draw function is called with unmodified scroll position...