Fix region_highlight entries leaking across accept/edit cycles

Before this change, the highlight module tracked only the most recent
plugin-owned entry in a scalar (_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT) and
relied on a parameter-expansion subtraction to remove it:

    region_highlight=("${(@)region_highlight:#$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT}")

That has two failure modes:

1. The tracked string is expanded as a zsh glob pattern. If the user's
   ZSH_AUTOSUGGEST_HIGHLIGHT_STYLE contains characters like `#`
   (e.g. `fg=#RRGGBB`), the entry is never removed and orphans
   accumulate in region_highlight across redraws. See #789.

2. Only one entry can be tracked at a time. When apply is called
   repeatedly without a successful reset (which happens under fast edits
   and widget-wrapping interactions), every apply overwrites the tracked
   reference and previous entries are orphaned.

Both manifest as stale suggestion colors bleeding onto accepted text,
typically in combination with zsh-syntax-highlighting (whose entries
interleave with ours in region_highlight).

Changes:

* Track every plugin-owned entry in an array
  (_ZSH_AUTOSUGGEST_OWNED_HIGHLIGHTS).

* On zsh 5.9+, tag each entry with `memo=zsh-autosuggestions` and on
  reset strip by memo in a single pass — robust regardless of how other
  plugins manipulate region_highlight. This matches the mechanism
  zsh-syntax-highlighting has used since 0.8.0.

* On zsh < 5.9, remove owned entries by literal string comparison
  (loop, not pattern expansion) to avoid the `#`-as-glob issue.

* _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT is retained and kept in sync for
  backwards compatibility with any external consumer.

Adds spec/highlight_spec.rb covering:
  - round-trip with a hex-colored style (regression for #789)
  - reset does not touch foreign region_highlight entries
  - accumulated orphans from repeated apply calls are all cleaned up

Supersedes #790 (which used `shift -p region_highlight` — incorrect
when another plugin has appended the most-recent entry).

Fixes #789, #698.

src/highlight.zsh

@@ -3,14 +3,77 @@
 # Highlighting                                                       #
 #--------------------------------------------------------------------#
 
+# `is-at-least` is autoloaded in src/start.zsh, but we may be called
+# before start.zsh runs (e.g. another plugin triggering us). Safe to
+# autoload here too — it's idempotent.
+autoload -Uz is-at-least
+
+# Array of every region_highlight entry this plugin has added. Using an
+# array (instead of a single scalar) ensures every entry we own can be
+# removed on reset even when:
+#   * another plugin appends to region_highlight between our apply and
+#     reset calls (e.g. zsh-syntax-highlighting),
+#   * multiple apply calls occur without an intervening successful reset
+#     (which can happen under rapid edits / widget wrapping edge cases).
+typeset -ga _ZSH_AUTOSUGGEST_OWNED_HIGHLIGHTS
+
+# Returns 0 if this zsh supports the `memo=` attribute on region_highlight
+# entries (zsh 5.9+). The result is cached on first call.
+_zsh_autosuggest_highlight_memo_support() {
+	typeset -g _ZSH_AUTOSUGGEST_MEMO_SUPPORT
+	if [[ -z "$_ZSH_AUTOSUGGEST_MEMO_SUPPORT" ]]; then
+		if is-at-least 5.9; then
+			_ZSH_AUTOSUGGEST_MEMO_SUPPORT=1
+		else
+			_ZSH_AUTOSUGGEST_MEMO_SUPPORT=0
+		fi
+	fi
+	(( _ZSH_AUTOSUGGEST_MEMO_SUPPORT ))
+}
+
 # If there was a highlight, remove it
 _zsh_autosuggest_highlight_reset() {
 	typeset -g _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 
-	if [[ -n "$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT" ]]; then
-		region_highlight=("${(@)region_highlight:#$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT}")
+	if (( $#_ZSH_AUTOSUGGEST_OWNED_HIGHLIGHTS == 0 )); then
 		unset _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
+		return
 	fi
+
+	if _zsh_autosuggest_highlight_memo_support; then
+		# Single pass: drop every region_highlight entry carrying our
+		# memo tag. Order-independent and robust against interleaving
+		# with other plugins' entries.
+		local entry
+		local -a kept=()
+		for entry in $region_highlight; do
+			[[ "$entry" != *memo=zsh-autosuggestions* ]] && kept+=("$entry")
+		done
+		region_highlight=("${kept[@]}")
+	else
+		# Fallback for zsh < 5.9: remove by literal string comparison.
+		# We intentionally do NOT use `${(@)array:#$needle}` because
+		# that treats $needle as a glob pattern — `#` in hex colors
+		# (e.g. `fg=#RRGGBB`) would be interpreted as pattern-matching
+		# syntax and the entry would never be removed.
+		# See https://github.com/zsh-users/zsh-autosuggestions/issues/789
+		local owned entry
+		for owned in $_ZSH_AUTOSUGGEST_OWNED_HIGHLIGHTS; do
+			local -a kept=()
+			local removed=0
+			for entry in $region_highlight; do
+				if (( ! removed )) && [[ "$entry" == "$owned" ]]; then
+					removed=1
+				else
+					kept+=("$entry")
+				fi
+			done
+			region_highlight=("${kept[@]}")
+		done
+	fi
+
+	_ZSH_AUTOSUGGEST_OWNED_HIGHLIGHTS=()
+	unset _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 }
 
 # If there's a suggestion, highlight it
@@ -18,8 +81,16 @@ _zsh_autosuggest_highlight_apply() {
 	typeset -g _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 
 	if (( $#POSTDISPLAY )); then
-		typeset -g _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT="$#BUFFER $(($#BUFFER + $#POSTDISPLAY)) $ZSH_AUTOSUGGEST_HIGHLIGHT_STYLE"
-		region_highlight+=("$_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT")
+		local entry="$#BUFFER $(($#BUFFER + $#POSTDISPLAY)) $ZSH_AUTOSUGGEST_HIGHLIGHT_STYLE"
+		if _zsh_autosuggest_highlight_memo_support; then
+			entry+=" memo=zsh-autosuggestions"
+		fi
+		region_highlight+=("$entry")
+		_ZSH_AUTOSUGGEST_OWNED_HIGHLIGHTS+=("$entry")
+
+		# Retained for backwards compatibility with anything reading
+		# this variable externally.
+		_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT="$entry"
 	else
 		unset _ZSH_AUTOSUGGEST_LAST_HIGHLIGHT
 	fi