`:stopinsert` does not clean up insert mode properly; completion window stays open

[happy-dude]

Describe the bug
:stopinsert does not clear completion windows that are open, leaving vim in a weird state.
See also: neovim/neovim#12976 and proposed patch: tjdevries/neovim@0cf14bb

To Reproduce
Detailed steps to reproduce the behavior:

1. Run vim --clean -u NONE
2. Edit a file like the following:

" hello
" helper
" howdy
" hunting

call timer_start(2000, { -> execute("stopinsert") })

" On this line, after sourcing this file, enter ins-completion mode within 2 seconds.
" After entering, you will not be able to type ever again in vim
" h

3. Call the following ex command: call timer_start(2000, { -> execute("stopinsert") })
4. Enter insert mode
5. Use <C-p> or <C-n> to enter completion mode and open the popup window

Expected behavior
:stopinsert should clear any popup and completion windows that are opened

Screenshots

Environment (please complete the following information):

vim --version                                                                                                       output-to-cloud-storage ✱
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Sep 17 2020 10:06:36)
macOS version
Included patches: 1-1701
Compiled by Homebrew
Huge version without GUI.  Features included (+) or not (-):
+acl               -farsi             +mouse_sgr         +tag_binary
+arabic            +file_in_path      -mouse_sysmouse    -tag_old_static
+autocmd           +find_in_path      +mouse_urxvt       -tag_any_white
+autochdir         +float             +mouse_xterm       -tcl
-autoservername    +folding           +multi_byte        +termguicolors
-balloon_eval      -footer            +multi_lang        +terminal
+balloon_eval_term +fork()            -mzscheme          +terminfo
-browse            +gettext           +netbeans_intg     +termresponse
++builtin_terms    -hangul_input      +num64             +textobjects
+byte_offset       +iconv             +packages          +textprop
+channel           +insert_expand     +path_extra        +timers
+cindent           +ipv6              +perl              +title
-clientserver      +job               +persistent_undo   -toolbar
+clipboard         +jumplist          +popupwin          +user_commands
+cmdline_compl     +keymap            +postscript        +vartabs
+cmdline_hist      +lambda            +printer           +vertsplit
+cmdline_info      +langmap           +profile           +virtualedit
+comments          +libcall           -python            +visual
+conceal           +linebreak         +python3           +visualextra
+cryptv            +lispindent        +quickfix          +viminfo
+cscope            +listcmds          +reltime           +vreplace
+cursorbind        +localmap          +rightleft         +wildignore
+cursorshape       +lua               +ruby              +wildmenu
+dialog_con        +menu              +scrollbind        +windows
+diff              +mksession         +signs             +writebackup
+digraphs          +modify_fname      +smartindent       -X11
-dnd               +mouse             -sound             -xfontset
-ebcdic            -mouseshape        +spell             -xim
+emacs_tags        +mouse_dec         +startuptime       -xpm
+eval              -mouse_gpm         +statusline        -xsmp
+ex_extra          -mouse_jsbterm     -sun_workshop      -xterm_clipboard
+extra_search      +mouse_netterm     +syntax            -xterm_save
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
       defaults file: "$VIMRUNTIME/defaults.vim"
  fall-back for $VIM: "/usr/local/share/vim"
Compilation: clang -c -I. -Iproto -DHAVE_CONFIG_H   -DMACOS_X -DMACOS_X_DARWIN  -g -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
Linking: clang   -L. -fstack-protector-strong -L/usr/local/lib -L/usr/local/opt/libyaml/lib -L/usr/local/opt/openssl@1.1/lib -L/usr/local/opt/readline/lib  -L/usr/local/lib -o vim        -lncurses -liconv -lintl -framework AppKit  -L/usr/local/opt/lua/lib -llua5.3 -mmacosx-version-min=10.15 -fstack-protector-strong -L/usr/local/lib  -L/usr/local/Cellar/perl/5.32.0/lib/perl5/5.32.0/darwin-thread-multi-2level/CORE -lperl -lm -lutil -lc  -L/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/config-3.8-darwin -lpython3.8 -lintl -framework CoreFoundation  -lruby.2.7

Additional context
See also: neovim/neovim#12976 and proposed patch: tjdevries/neovim@0cf14bb

[chrisbra]

fixed by d0e1b71