Weird arrow key behaviour in fzf.vim terminal mode

Question

My .vimrc contains junegunn's implementation of Most Recently Used FZF search:

nmap <Leader><Leader> :FZFMru<CR>

command! FZFMru call fzf#run({
\  'source':  v:oldfiles,
\  'sink':    'e',
\  'options': '-m -x +s',
\  'down':    '40%'})

Recently, while this FZF window is open, I find the up and down keys behave strangely. In normal mode I hit <Leader><Leader>, and the FZF windows opens. Hitting <Up>, <Down>, <Left> or <Right> switches me into insert mode and types respectively A, B, C or D (upper case). Looking at :map I don't see that these keys are mapped to anything, in any mode.

What is going on?

Update: it seems like this is connected to the fzf.vim plugin's using terminal mode - in the floating terminal the arrow keys are mapped to these sequences:

up - "\027[A"
down - "\027[B"
left - "\027[D"
right - "\027[C"

(ie vi-compatible mode, even though within my main Vim session I use set nocompatible and this isn't an issue.) There is a GitHub discussion here. However, it's very hard to figure out under which conditions this occurs.

Answer 1

This problem was caused by a change I'd made in my .vimrc, a mapping to let me <Esc> to exit terminal mode tnoremap <Esc> <C-\><C-n>:bd!<CR>, identical to this GitHub issue. One solution is to map double escape, eg tnoremap <Esc><Esc> <c-\><c-n>.

Explanation: the issue seems to be that: a) fzf.vim uses terminal mode, which I rarely use; b) I added a terminal-mode modification which only showed up when using this plugin; c) apparently when <Esc> is mapped, Vim temporarily interprets it as a literal escape sequence, so that the arrow keys send ^[OA, etc., which goes from terminal mode back to normal mode, adds a new line and types A. (All of this is explained in many other places, such as here.)