The bottom horizontal scroll bar is a very important feature.
It is there because the theme width of the page being edited is wider than the available space on the user’s screen.
The default is to show the page at the theme width.
There is an option on the “Admin Options” tab called : Disable Page/Post Editor Styling that will disable this behavior. But do note that the Visual Editor will then not correspond as closely to the actual layout of the page or post.
Just to note, Weaver II is becoming EXTREMELY outdated. We highly recommend to abandon using it, and whenever possible, converting to Weaver Xtreme.