TextEdit app is displaying wide borders at bottom and right corners
The bars shown are scroll bars. Go to System Preferences app → General and under Show scroll bars: select When scrolling.



It's likely that either Always or Automatically based on mouse or trackpad option is selected, which is causing the scroll bars to appear. 

With When scrolling option selected, the wide borders will no longer be shown. The scroll bars, when displayed will be narrow, translucent and will disappear moments after the scrolling is complete.