Neovim version (nvim -v)

NVIM v0.6.0-dev+373-g36408146a

Vim (not Nvim) behaves the same?

no, vim 8.2.3441

Operating system/version

Arch Linux

Terminal name/version

kitty 0.23.1

$TERM environment variable

xterm-kitty

Installation

Build from source

How to reproduce the issue

1. Run nvim --clean
2. Add mappings :nnoremap <Esc> <Cmd>echomsg 'Esc'<CR> and :nnoremap x <Cmd>echomsg 'x'<CR>.
3. Press ALT-x or press <Esc> and x quickly
4. Run :messages

Expected behavior

There are two lines of messages:

Esc
x

Doing the same things in Vim in terminal shows these two messages.

Actual behavior

There are no messages

Since a key typed too fast following <Esc> in terminal is indistinguishable from an ALT keystroke, I think it is reasonable that mappings should be applied to the <Esc> and the following key just like they are applied to normally typed keys if the ALT- keystroke is unmapped.