Editing a current writer document ( even punctuation, a comma, or space deletion) sometimes causes window to go blank for up to a minute. Why? Annoying!
You ask Why and don’t provide an minimum of information about LibreOffice version, operating system and its version. Why?
But anyway: If set, try to disable OpenGL in Tools -> Options -> LibreOffice -> View -> Category: Graphics Output -> Option: [ ] Use OpenGL for all rendering and check whether that fixes your problem
Also, how old is your computer? Is your hard drive nearly full? How much RAM memory does the computer have?