diff --git a/src/plugins/texteditor/textdocument.cpp b/src/plugins/texteditor/textdocument.cpp
index cb421f2bf7c..2636ad46e23 100644
--- a/src/plugins/texteditor/textdocument.cpp
+++ b/src/plugins/texteditor/textdocument.cpp
@@ -914,19 +914,23 @@ bool TextDocument::reload(QString *errorString, ReloadFlag flag, ChangeType type
 void TextDocument::resetSyntaxHighlighter(const std::function<SyntaxHighlighter *()> &creator,
                                           bool threaded)
 {
-    if (d->m_highlighterRunner)
-        delete d->m_highlighterRunner;
+    delete d->m_highlighterRunner;
 
-    static const bool envValue
-        = qtcEnvironmentVariable("QTC_USE_THREADED_HIGHLIGHTER", "TRUE").toUpper()
-          == QLatin1String("TRUE");
+    static const std::optional<bool> envValue = []() -> std::optional<bool> {
+        const QString key("QTC_USE_THREADED_HIGHLIGHTER");
+        if (qtcEnvironmentVariableIsSet(key)) {
+            const QString value = qtcEnvironmentVariable(key).toUpper();
+            return value != "FALSE" && value != "0";
+        }
+        return {};
+    }();
 
     SyntaxHighlighter *highlighter = creator();
     highlighter->setFontSettings(TextEditorSettings::fontSettings());
     highlighter->setMimeType(mimeType());
     d->m_highlighterRunner = new SyntaxHighlighterRunner(highlighter,
                                                          document(),
-                                                         threaded && envValue);
+                                                         envValue.value_or(threaded));
 }
 
 void TextDocument::cleanWhitespace(const QTextCursor &cursor)
diff --git a/src/plugins/texteditor/textdocument.h b/src/plugins/texteditor/textdocument.h
index ad3fe262e24..5879d9e9ec2 100644
--- a/src/plugins/texteditor/textdocument.h
+++ b/src/plugins/texteditor/textdocument.h
@@ -127,7 +127,7 @@ public:
     QTextDocument *document() const;
 
     using SyntaxHighLighterCreator = std::function<SyntaxHighlighter *()>;
-    void resetSyntaxHighlighter(const SyntaxHighLighterCreator &creator, bool threaded = true);
+    void resetSyntaxHighlighter(const SyntaxHighLighterCreator &creator, bool threaded = false);
     SyntaxHighlighterRunner *syntaxHighlighterRunner() const;
 
     bool reload(QString *errorString, QTextCodec *codec);