Work around a crashing bug in Zsh

See [zsh-workers 48936](https://www.zsh.org/mla/workers/2021/msg01162.html).
Fixes issue #274.

module/.autocomplete.async

@@ -107,6 +107,11 @@ bindkey -s -M menuselect '^S' '^G^S'
 .autocomplete.async.complete() {
   .autocomplete.zle-flags $LASTWIDGET
 
+  [[ -v MENUSELECT && ( -z $KEYS || $( bindkey "$KEYS" ) != *'" '$LASTWIDGET ) ]] &&
+      return
+
+  unset MENUSELECT
+
   (( KEYS_QUEUED_COUNT || PENDING )) &&
     return 0
 
@@ -117,11 +122,6 @@ bindkey -s -M menuselect '^S' '^G^S'
   [[ -v functions[_zsh_autosuggest_highlight_apply] ]] &&
     _zsh_autosuggest_highlight_apply
 
-  [[ -v MENUSELECT && $( bindkey "$KEYS" ) != *'" '$LASTWIDGET ]] &&
-      return
-
-  unset MENUSELECT
-
   if (( REGION_ACTIVE )) ||
       [[ -v _autocomplete__isearch && $LASTWIDGET == *(incremental|isearch)* ]]; then
     zle -Rc

module/.autocomplete.key-binding

@@ -8,7 +8,9 @@ zmodload -Fa zsh/parameter p:funcstack p:functions
     .autocomplete.key
 
   local tab_style; zstyle -s :autocomplete:tab: widget-style tab_style ||
-    tab_style='complete-word'
+      tab_style='complete-word'
+  local backtab_style; zstyle -s :autocomplete:shift-tab: widget-style backtab_style ||
+      backtab_style=${tab_style:/menu-complete/reverse-menu-complete}
 
   if zstyle -t :autocomplete:tab: fzf || zstyle -t :autocomplete:tab: fzf-completion; then
     typeset -gH fzf_default_completion=$tab_style
@@ -28,15 +30,17 @@ zmodload -Fa zsh/parameter p:funcstack p:functions
     bindkey -M emacs $key[Tab] $tab_style
     bindkey -M viins $key[Tab] $tab_style
   fi
-  local backtab_style=${tab_style:/menu-complete/reverse-menu-complete}
   bindkey -M emacs $key[Shift-Tab] $backtab_style
   bindkey -M viins $key[Shift-Tab] $backtab_style
 
-  if [[ $tab_style == menu-* ]]; then
+  if [[ $tab_style == *menu-* ]]; then
     bindkey -M menuselect $key[Tab] menu-complete
-    bindkey -M menuselect $key[Shift-Tab] reverse-menu-complete
   else
     bindkey -M menuselect $key[Tab] accept-line
+  fi
+  if [[ $backtab_style == *menu-* ]]; then
+    bindkey -M menuselect $key[Shift-Tab] reverse-menu-complete
+  else
     bindkey -M menuselect $key[Shift-Tab] send-break
   fi
 }

widget/.autocomplete.complete-word.post

@@ -1,33 +1,42 @@
 #autoload
+unset MENUSELECT
 unset curcontext
+compstate[insert]=
 compstate[list]=
 
+local key_name=${(kL)key[(Re)$KEYS]}
+local key_style; zstyle -s :autocomplete:${key_name}: widget-style key_style
+
 if  ( [[ $KEYS == $key[Shift-Tab] ]] ||
-      zstyle -t ":autocomplete:${(kL)key[(Re)$KEYS]}:" insert-unambiguous
+      zstyle -t ":autocomplete:${key_name}:" insert-unambiguous
     ) && [[ $IPREFIX$PREFIX$SUFFIX$ISUFFIX != *${compstate[unambiguous]:#?}* ]]; then
-  compstate[insert]=${${(M)WIDGET:#*menu-*}:+automenu-}unambiguous
+  # Work around a crashing bug in Zsh.
+  # See [zsh-workers 48936](https://www.zsh.org/mla/workers/2021/msg01162.html).
+  [[ $key_style == *menu-* && $KEYS == ($key[Tab]|$key[Shift-Tab]) ]] &&
+      compstate[insert]='automenu-'
+
+  compstate[insert]+=unambiguous
   return
 fi
 
-compstate[insert]=
-unset MENUSELECT
-
 if [[ $WIDGET == *menu-select ]]; then
   # Determine which terminal line we're on (for async completion).
   (( _autocomplete__buffer_start_line = max( BUFFERLINES, LINES - compstate[list_lines] ) ))
   MENUSELECT=0
 fi
 
+# Work around a crashing bug in Zsh.
+# See [zsh-workers 48936](https://www.zsh.org/mla/workers/2021/msg01162.html).
+[[ $key_style == *menu-* && $KEYS == ($key[Tab]|$key[Shift-Tab]) ]] &&
+    compstate[insert]='menu:'
+
 if [[ $KEYS == $key[Shift-Tab] ]]; then
-  compstate[insert]+=0
+  compstate[insert]+='0'
 else
-  compstate[insert]+=1
+  compstate[insert]+='1'
 fi
 
 local -a tags; zstyle -a :autocomplete: add-space tags ||
-  tags=( executables aliases functions builtins reserved-words commands )
-if [[ $RBUFFER != [[:space:]]* && -n ${${=${_comp_tags:-$_lastcomp[tags]}}:*tags} ]]; then
-  [[ $WIDGET == *menu-* ]] &&
-      compstate[insert]="menu:$compstate[insert]"
-  compstate[insert]+=' '
-fi
+    tags=( executables aliases functions builtins reserved-words commands )
+[[ $RBUFFER != [[:space:]]* && -n ${${=${_comp_tags:-$_lastcomp[tags]}}:*tags} ]] &&
+    compstate[insert]+=' '