For this version of RStudtion on MAC v10.14.6, there is a bug for scroll bars both vertical and horizontal bars.
It seems the scroll bar was set to have a higher priority to be rendered over other windows/objects.
Also, the horizontal scroll bar in the code window is over the text on the last line. The text should be above the scroll bar.
Please kindly check the scroll bars in all windows. Thank you very much in advance.