Pasting text on Vim inside tmux breaks indentation

Question

When pasting text into Vim without tmux, it shows a normal paste:

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 

Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. 

Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. 

However, when using Vim inside tmux, the indentation breaks and the paste shows like this:

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 

  Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. 

    Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. 

How can this be solved?

Answer 1

So using :set paste first can resolv the issue.

as you asked here is my tmuxrc but it is very short :

:~$ cat ~/.tmuxrc
set -g status-utf8 on
:~$

So I don't think it is special

Answer 2

Manually toggling :set paste/:set nopaste as suggested by francois P is cumbersome, and resetting the TERM variable as evaristegd suggests is a very bad idea as explained in the comments (which hint at the better solution below). The best solutions that I could find are explained here, and I'll briefly repeat them below.

Background

The central concept for this problem is "bracketed paste mode", which lets a program distinguish between pasted and typed input. The problem is that vim 8 only enables bracketed paste mode when started with certain values for the TERM-variable (e.g. TERM=xterm-256color). Within tmux however, vim should get either TERM=screen (default) or TERM=tmux, and in neither case does vim enable bracketed paste mode (I have no idea why not, please edit if you do).

Solution

Add the following to your ~/.vimrc, as explained in the vim help: :h xterm-bracketed-paste.

if &term =~ "screen"                                                   
    let &t_BE = "\e[?2004h"                                              
    let &t_BD = "\e[?2004l"                                              
    exec "set t_PS=\e[200~"                                              
    exec "set t_PE=\e[201~"                                              
endif