don't highlight in isearch if ISEARCH_ACTIVE is unsupported

Old versions of zsh don't expose ISEARCH_ACTIVE. Therefore we
are unable to re-apply zle_highlight on top and it is impossible
to see the underlined area.

Completely disable highlighting in isearch in that case.

To do that, we need to make sure we are actually called when
something changes in isearch.

Trumps #257.

Author: m0vie

README.md

@@ -32,6 +32,15 @@ custom widgets have been created (i.e., after all `zle -N` calls and after
 running `compinit`).  Widgets created later will work, but will not update the
 syntax highlighting.
 
+### Why does syntax highlighting not work while searching history?
+
+In `zsh` versions before v5.3 it is not possible for `zsh-syntax-highlighting.zsh`
+to know if an incremental search is currently active and that matched parts of the
+buffer should be underlined (or otherwise highlighted). Therefore, it is not possible
+for `zsh-syntax-highlighting.zsh` to apply syntax highlighting and to underline the
+matched part of the search. While searching the history, the latter is more important,
+so syntax highlighting is disabled in this case.
+
 ### How are new releases announced?
 
 There is currently no "push" announcements channel.  However, the following

zsh-syntax-highlighting.zsh

@@ -62,6 +62,12 @@ _zsh_highlight()
   # Store the previous command return code to restore it whatever happens.
   local ret=$?
 
+  # Do not highlight in isearch if ISEARCH_ACTIVE is unsupported (zsh < 5.3).
+  if [[ $WIDGET == zle-isearch-update ]] && ! (( $+ISEARCH_ACTIVE )); then
+    region_highlight=()
+    return $ret
+  fi
+
   setopt localoptions warncreateglobal
   setopt localoptions noksharrays
   local REPLY # don't leak $REPLY into global scope
@@ -313,6 +319,10 @@ _zsh_highlight_bind_widgets || {
 # E.g. remove cursor imprint, don't highlight partial paths, ...
 _zsh_highlight_set_or_wrap_special_zle_widget zle-line-finish
 
+# Always wrap special zle-isearch-update widget to be notified of updates in isearch
+_zsh_highlight_set_or_wrap_special_zle_widget zle-isearch-update
+
+
 # Resolve highlighters directory location.
 _zsh_highlight_load_highlighters "${ZSH_HIGHLIGHT_HIGHLIGHTERS_DIR:-${${0:A}:h}/highlighters}" || {
   echo 'zsh-syntax-highlighting: failed loading highlighters, exiting.' >&2