Need to initialize tracking value on mousedown.
diff --git a/controls/scroll.c b/controls/scroll.c
index 209f762..8a6c3cc 100644
--- a/controls/scroll.c
+++ b/controls/scroll.c
@@ -1023,6 +1023,9 @@
SCROLL_TrackingWin = hwnd;
SCROLL_TrackingBar = nBar;
SCROLL_TrackingPos = trackThumbPos + lastMousePos - lastClickPos;
+ SCROLL_TrackingVal = SCROLL_GetThumbVal( infoPtr, &rect,
+ vertical,
+ SCROLL_TrackingPos );
if (!SCROLL_MovingThumb)
SCROLL_DrawMovingThumb(hdc, &rect, vertical, arrowSize, thumbSize);
}