Unable to scroll a page in edit mode if the cursor is on top of the page

Steps to reproduce:

When editing a page, if the mouse cursor is placed on the scroll bar AND right beside of the page title section, then the scrolling becomes non-functional; instead it will highlight the page title section. On the other hand, if the mouse cursor is placed below the page title section then the scrolling is functional.

I suspect issue is caused by the mouse is focusing the page title section instead of the scroll bar, even the though the scroll bar is shown on top of the page title section.