'driver': Don't highlight in isearch

zsh version 5.2 and lower don't support ISEARCHMATCH_ACTIE and
we are unable to re-apply zle_highlight on top. Therefore it is
impossible to see the underlined matched area.

Since that information is more important, 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.

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 5.3 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

@@ -58,6 +58,14 @@ _zsh_highlight()
   # Store the previous command return code to restore it whatever happens.
   local ret=$?
 
+  # Remove all highlighting in isearch, so that only the underlining done by zsh itself remains.
+  # For details see FAQ entry 'Why does syntax highlighting not work while searching history?'
+  # and http://www.zsh.org/mla/users/2016/msg00415.html.
+  if [[ $WIDGET == zle-isearch-update ]]; then
+    region_highlight=()
+    return $ret
+  fi
+
   setopt localoptions warncreateglobal
   setopt localoptions noksharrays
   local REPLY # don't leak $REPLY into global scope
@@ -243,6 +251,10 @@ _zsh_highlight_bind_widgets()
   # E.g. remove cursor imprint, don't highlight partial paths, ...
   widgets_to_bind+=(zle-line-finish)
 
+  # Always wrap special zle-isearch-update widget to be notified of updates in isearch.
+  # This is needed because we need to disable highlighting in that case.
+  widgets_to_bind+=(zle-isearch-update)
+
   local cur_widget
   for cur_widget in $widgets_to_bind; do
     case $widgets[$cur_widget] in