现象
在使用Vim的补全的时候，可能从其他IDE/编辑器过来的同学可能会对Vim在弹出补全菜单后，Tab和Enter的行为感到不理解，例如经常会直接插进一个制表符或者换行回车，这其实是Vim自己在插入模式下补全功能的特殊设计，至于为什么这么设计，那就得问开发者了:)
原因
以下摘自Vim的帮助文档 :help popupmenu-completion，简单来说就是Vim的插入模式处于补全状态时，其实有3个状态，不同状态下Enter的功能表现是不一样的，而Tab在Vim补全的上下文和Space几乎是一样的，就是插入对应的空格/制表符
There are three states:
- A complete match has been inserted, e.g., after using CTRL-N or CTRL-P.
- A cursor key has been used to select another match. The match was not inserted then, only the entry in the popup menu is highlighted.
- Only part of a match has been inserted and characters were typed or the backspace key was used. The list of matches was then adjusted for what is in front of the cursor.
In all three states these can be used: CTRL-Y Yes: Accept the currently selected match and stop completion. CTRL-E End completion, go back to what was there before selecting a match (what was typed or longest common string).
Select a match several entries back, but don't insert it. Select a match several entries further, but don't insert it. Select the previous match, as if CTRL-P was used, but don't insert it. Select the next match, as if CTRL-N was used, but don't insert it. or Stop completion without changing the match and insert the typed character. The behavior of the key depends on the state you are in: first state: Use the text as it is and insert a line break. second state: Insert the currently selected match. third state: Use the text as it is and insert a line break. In other words: If you used the cursor keys to select another entry in the list of matches then the key inserts that match. If you typed something else then inserts a line break.
建议
Vim的这种设计也许确实让习惯了其他IDE/Editor的小伙伴们难以适应，就个人经验而言，Vim的这种模式其实还是很容易适应的，尤其是你较少使用方向键，手长时间放在home row上时，使用Ctrl-N/P进行上下选择的补全方式可以让你减少一次Enter的击键，习惯了以后其实还是蛮有快感的。
但习惯这种东西毕竟因人而异，如果你确实不习惯这种补全方式，或者你经常多个IDE混合使用，希望能有一致的手感和体验，那我的建议是可以参考一下CoC的设置，其对应的关键builtin函数是 pumvisible()，这里我就不班门弄斧了，因为我个人并不是这种使用方式，有兴趣的同学可以在评论区交流:)
" Use tab for trigger completion with characters ahead and navigate
" NOTE: There's always complete item selected by default, you may want to enable
" no select by `"suggest.noselect": true` in your configuration file
" NOTE: Use command ':verbose imap <tab>' to make sure tab is not mapped by
" other plugin before putting this into your config
inoremap <silent><expr> <TAB>
\ coc#pum#visible() ? coc#pum#next(1) :
\ CheckBackspace() ? "\<Tab>" :
\ coc#refresh()
inoremap <expr><S-TAB> coc#pum#visible() ? coc#pum#prev(1) : "\<C-h>"
参考
:help ins-completion-menu:help popupmenu-completion:help pumvisible- coc.nvim
