Vim map esc key in insert mode without losing undo sequence?

Question

I have one question about Vim. I understood map as just the equal command till now. Here's the thing. I put imap <co> <esc>o in my .vimrc file to start a new line. As to the reason why not just map <CR> is that I followed the suggestion by Drew Neil that using esc and o command to replace so that I can use undo command line by line. However, when I use <co> to start a line, the undo command doesn't work, which means Vim doesn't switch the mode even I map the <esc> ! I don't know whether is feature or bug of Vim? I will appreciate it if you help me in some ways.

Answer 1

You can use inoremap <co> <cg>u<esc>o to get the desired behaviour you want. <cg>u breaks the current undo sequence (see :h i_CTRL-G_u ).

I actually never thought about why having <esc> in insert mappings doesn't count as breaking the undo sequence. I could certainly theorize why but doing so is dangerous in an SO answer, so I won't :)

Edited to add: Simpler would be: inoremap <co> <cg>u<cr>