vim c-x c-o completion appears in a new window

Question

As shown in the picture, when using c-x c-o to complete, a new window will appear above the window, which will still exist after completion.

c-n There is no completion.

How to avoid the upper window appearing like c-n

Answer 1

That window is called preview window, which can be controlled by setting completeopt. It can be disabled by setting the following:
set completeopt-=preview

You can also close any preview window via C-w z

My settings are:
set completeopt=longest,menuone

Detailed instructions can be found at:
help:completeopt
help:preview-window