This is a bug I haven't experienced in forever, but it seems to have crept up again with one of the recent releases.  The screenshots say it all, let me know if you have any questions.  As soon as the grid exceeds 1100 or so pixels wide the scroll bar disappears. 

 

 

Edit: changing the grid width resolves the issues SOMETIMES.  At other times it does not.  Sometimes I have to resize the entire IQ window so it's not maximized to get the scroll bar back.

 

This is a 4K monitor with no scaling, in case that's one of the variables.