https://bugs.gentoo.org/921490
https://www.sqlite.org/forum/forumpost/d16aeb397d

https://github.com/kovidgoyal/calibre/commit/0a23fabd5b4bff152047ba0cffbc2b67592645d1
https://github.com/kovidgoyal/calibre/commit/3349979551db05ec0c8754d7ea5a4e57a97f8a0a
https://github.com/kovidgoyal/calibre/commit/da82b673f09efbe9cdcc1e0900f0a9f04d8de052
--- a/ext/fts5/fts5_aux.c
+++ b/ext/fts5/fts5_aux.c
@@ -209,10 +209,18 @@
       rc = fts5CInstIterNext(&p->iter);
     }
   }
 
   if( iPos==p->iRangeEnd ){
+    if( p->bOpen ){
+      if( p->iter.iStart>=0 && iPos>=p->iter.iStart ){
+        fts5HighlightAppend(&rc, p, &p->zIn[p->iOff], iEndOff - p->iOff);
+        p->iOff = iEndOff;
+      }
+      fts5HighlightAppend(&rc, p, p->zClose, -1);
+      p->bOpen = 0;
+    }
     fts5HighlightAppend(&rc, p, &p->zIn[p->iOff], iEndOff - p->iOff);
     p->iOff = iEndOff;
   }
 
   return rc;

ADDED   ext/fts5/test/fts5tokenizer2.test
Index: ext/fts5/test/fts5tokenizer2.test
==================================================================
--- /dev/null
+++ b/ext/fts5/test/fts5tokenizer2.test
@@ -0,0 +1,89 @@
+# 2023 Nov 03
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+#
+# Tests focusing on the built-in fts5 tokenizers. 
+#
+
+source [file join [file dirname [info script]] fts5_common.tcl]
+set testprefix fts5tokenizer2
+
+# If SQLITE_ENABLE_FTS5 is defined, omit this file.
+ifcapable !fts5 {
+  finish_test
+  return
+}
+
+sqlite3_fts5_create_tokenizer db tst get_tst_tokenizer
+proc get_tst_tokenizer {args} {
+  return "tst_tokenizer"
+}
+proc tst_tokenizer {flags txt} {
+  set token ""
+  set lTok [list]
+
+  foreach c [split $txt {}] {
+    if {$token==""} {
+      append token $c
+    } else {
+      set t1 [string is upper $token]
+      set t2 [string is upper $c]
+
+      if {$t1!=$t2} {
+        lappend lTok $token
+        set token ""
+      }
+      append token $c
+    }
+  }
+  if {$token!=""} { lappend lTok $token }
+
+  set iOff 0
+  foreach t $lTok {
+    set n [string length $t]
+    sqlite3_fts5_token $t $iOff [expr $iOff+$n]
+    incr iOff $n
+  }
+}
+
+do_execsql_test 1.0 {
+  CREATE VIRTUAL TABLE t1 USING fts5(t, tokenize=tst);
+}
+
+do_execsql_test 1.1 {
+  INSERT INTO t1 VALUES('AAdontBBmess');
+}
+
+do_execsql_test 1.2 {
+  SELECT snippet(t1, 0, '>', '<', '...', 4) FROM t1('BB');
+} {AAdont>BB<mess}
+
+do_execsql_test 1.3 {
+  SELECT highlight(t1, 0, '>', '<') FROM t1('BB');
+} {AAdont>BB<mess}
+
+do_execsql_test 1.4 {
+  SELECT highlight(t1, 0, '>', '<') FROM t1('AA');
+} {>AA<dontBBmess}
+
+do_execsql_test 1.5 {
+  SELECT highlight(t1, 0, '>', '<') FROM t1('dont');
+} {AA>dont<BBmess}
+
+do_execsql_test 1.6 {
+  SELECT highlight(t1, 0, '>', '<') FROM t1('mess');
+} {AAdontBB>mess<}
+
+do_execsql_test 1.7 {
+  SELECT highlight(t1, 0, '>', '<') FROM t1('BB mess');
+} {AAdont>BBmess<}
+
+
+finish_test