As shown in the figure, use
c-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:
You can also close any preview window through c-w z.
My settings are:
Details can be found in: