You need to know how wide the table is and how wide the scrollbar is. Here's some code you could add to the colview project that ships with CVI
if (event == EVENT_LEFT_DOUBLE_CLICK)
{
int nTableWidth;
int nScrollBarWidth;
GetCtrlAttribute (panel, MAINPNL_RESULTS, ATTR_SCROLL_BAR_SIZE, &nScrollBarWidth);
GetCtrlAttribute (panel, MAINPNL_RESULTS, ATTR_WIDTH, &nTableWidth);
// for EVENT_LEFT_DOUBLE_CLICK, eventData2 = mouse horizontal position
if (eventData2 >= (nTableWidth - nScrollBarWidth))
MessagePopup ("DoubleClick", "Double click on scrollbar");
}