I have just updated from 1.2.5042-1 to the new stable release (1.3.959-1). Precisely since the update, I am missing the horizontal and vertical scrollbars in the source panel. All the other panels still have the scrollbars.
On GitHub I see that there is an issue on the Mac platform with version 1.3.898 :
No scrollbar in source editor on Mac desktop #6432
Is that your platform?
Yes, I am on macOS 10.15.5. Looks like that is exactly the bug that I encounter - I am also using the Magic Mouse 2 and the trackpad.