A new window appears for vim c-x c-o completions.

  question, vim

请输入图片描述

As shown in the figure, usec-x c-oWhen completing, a new window will appear at the top of the window and will still exist after completing.

There is no c-n completion.

How to avoid the top window like c-n

The window is called preview window, which can be controlled by setting completeopt, and can be disabled by the following settings:
set completeopt-=preview

You can also close any preview window through c-w z.

My settings are:
set completeopt=longest,menuone

Details can be found in:
help:completeopt
help:preview-window