Use the new, unreleased zsh 'memo=' feature to remove only our own entries from $region_highlight.

Fixes #418 (interoperability issue with other plugins).

zsh-syntax-highlighting.zsh

@@ -83,12 +83,63 @@ _zsh_highlight()
     return $ret
   }
 
+  # Probe the memo= feature, once.
+  (( ${+zsh_highlight__memo_feature} )) || {
+    region_highlight+=( " 0 0 fg=red, memo=zsh-syntax-highlighting" )
+    case ${region_highlight[-1]} in
+      ("0 0 fg=red")
+        # zsh 5.8 or earlier
+        integer -gr zsh_highlight__memo_feature=0
+        ;;
+      ("0 0 fg=red memo=zsh-syntax-highlighting")
+        # zsh 5.9 or later
+        integer -gr zsh_highlight__memo_feature=1
+        ;;
+      (" 0 0 fg=red, memo=zsh-syntax-highlighting") ;&
+      (*)
+        # We can get here in two ways:
+        #
+        # 1. When not running as a widget.  In that case, $region_highlight is
+        # not a special variable (= one with custom getter/setter functions
+        # written in C) but an ordinary one, so the third case pattern matches
+        # and we fall through to this block.  (The test suite uses this codepath.)
+        #
+        # 2. When running under a future version of zsh that will have changed
+        # the serialization of $region_highlight elements from their underlying
+        # C structs, so that none of the previous case patterns will match.
+        #
+        # In either case, fall back to a version check.
+        #
+        # The memo= feature was added to zsh in commit zsh-5.8-172-gdd6e702ee.
+        # The version number at the time was 5.8.0.2-dev (see Config/version.mk).
+        # Therefore, on 5.8.0.3 and newer the memo= feature is available.
+        #
+        # On zsh version 5.8.0.2 between the aforementioned commit and the
+        # first Config/version.mk bump after it (which, at the time of writing,
+        # is yet to come), this condition will false negative.
+        if is-at-least 5.8.0.3; then
+          integer -gr zsh_highlight__memo_feature=1
+        else
+          integer -gr zsh_highlight__memo_feature=0
+        fi
+        ;;
+    esac
+    region_highlight[-1]=()
+  }
+
+  # Reset region_highlight to build it from scratch
+  if (( zsh_highlight__memo_feature )); then
+    region_highlight=( "${(@)region_highlight:#*memo=zsh-syntax-highlighting*}" )
+  else
+    # Legacy codepath.  Not very interoperable with other plugins (issue #418).
+    region_highlight=()
+  fi
+
   # 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?'.
   # This disables highlighting during isearch (for reasons explained in README.md) unless zsh is new enough
   # and doesn't have the pattern matching bug
   if [[ $WIDGET == zle-isearch-update ]] && { $zsh_highlight__pat_static_bug || ! (( $+ISEARCHMATCH_ACTIVE )) }; then
-    region_highlight=()
     return $ret
   fi
 
@@ -124,9 +175,6 @@ _zsh_highlight()
   # Do not highlight if there are pending inputs (copy/paste).
   [[ $PENDING -gt 0 ]] && return $ret
 
-  # Reset region highlight to build it from scratch
-  region_highlight=();
-
   {
     local cache_place
     local -a region_highlight_copy
@@ -245,7 +293,7 @@ _zsh_highlight_apply_zle_highlight() {
   else
     start=$second end=$first
   fi
-  region_highlight+=("$start $end $region")
+  region_highlight+=("$start $end $region, memo=zsh-syntax-highlighting")
 }
 
 
@@ -285,7 +333,7 @@ _zsh_highlight_add_highlight()
   shift 2
   for highlight; do
     if (( $+ZSH_HIGHLIGHT_STYLES[$highlight] )); then
-      region_highlight+=("$start $end $ZSH_HIGHLIGHT_STYLES[$highlight]")
+      region_highlight+=("$start $end $ZSH_HIGHLIGHT_STYLES[$highlight], memo=zsh-syntax-highlighting")
       break
     fi
   done