Respect the force argument in DOMTokenList.toggle()

src/changes/changes.xml

@@ -8,6 +8,10 @@
 
     <body>
         <release version="5.8.0" date="xxxx xx, 2026" description="Chrome/Edge 157, Bugfixes">
+            <action type="fix" dev="rbri" issue="#1188" due-to="Vladimir Babin">
+                DOMTokenList.toggle() now honors the optional force argument: a true value only adds the token
+                and a false value only removes it, matching the DOM standard.
+            </action>
             <action type="add" dev="rbri" issue="#638">
                 Content-Security-Policy: XMLHttpRequest supports the connect-src directive (with default-src as fallback);
                 the request url and all redirect targets are checked, blocked requests fail with a network error.

src/main/java/org/htmlunit/javascript/host/dom/DOMTokenList.java

@@ -277,10 +277,12 @@ public boolean replace(final String oldToken, final String newToken) {
     /**
      * Toggles the token, adding or removing it.
      * @param token the token to add or remove
+     * @param force if {@code true}, only adds the token; if {@code false}, only removes it;
+     *        if not specified, the token is toggled
      * @return whether the string now contains the token or not
      */
     @JsxFunction
-    public boolean toggle(final String token) {
+    public boolean toggle(final String token, final Object force) {
         if (org.htmlunit.util.StringUtils.isEmptyOrNull(token)) {
             throw JavaScriptEngine.asJavaScriptException(
                     getWindow(),
@@ -295,15 +297,18 @@ public boolean toggle(final String token) {
         }
 
         final List<String> parts = split(getValue());
-        if (parts.contains(token)) {
+        final boolean present = parts.contains(token);
+        final boolean add = JavaScriptEngine.isUndefined(force) ? !present : JavaScriptEngine.toBoolean(force);
+
+        if (add && !present) {
+            parts.add(token);
+            updateAttribute(String.join(" ", parts));
+        }
+        else if (!add && present) {
             parts.remove(token);
             updateAttribute(String.join(" ", parts));
-            return false;
         }
-
-        parts.add(token);
-        updateAttribute(String.join(" ", parts));
-        return true;
+        return add;
     }
 
     /**

src/test/java/org/htmlunit/javascript/host/dom/DOMTokenListTest.java

@@ -1760,6 +1760,85 @@ public void toggleNotDefined() throws Exception {
         toggle(null, "a");
     }
 
+    /**
+     * @throws Exception if the test fails
+     */
+    @Test
+    @Alerts({"a", "1", "true", "2", "a\\sb",
+             "<div\\sid=\"d1\"\\sclass=\"a\\sb\"></div>",
+             "class\\schanged\\sold:\\sa"})
+    public void toggleForceTrueAdds() throws Exception {
+        toggle("a", "b", "true");
+    }
+
+    /**
+     * @throws Exception if the test fails
+     */
+    @Test
+    @Alerts({"a\\sb", "2", "true", "2", "a\\sb",
+             "<div\\sid=\"d1\"\\sclass=\"a\\sb\"></div>"})
+    public void toggleForceTrueKeeps() throws Exception {
+        toggle("a b", "b", "true");
+    }
+
+    /**
+     * @throws Exception if the test fails
+     */
+    @Test
+    @Alerts({"a\\sb", "2", "false", "1", "a",
+             "<div\\sid=\"d1\"\\sclass=\"a\"></div>",
+             "class\\schanged\\sold:\\sa\\sb"})
+    public void toggleForceFalseRemoves() throws Exception {
+        toggle("a b", "b", "false");
+    }
+
+    /**
+     * @throws Exception if the test fails
+     */
+    @Test
+    @Alerts({"a", "1", "false", "1", "a",
+             "<div\\sid=\"d1\"\\sclass=\"a\"></div>"})
+    public void toggleForceFalseKeeps() throws Exception {
+        toggle("a", "b", "false");
+    }
+
+    private void toggle(final String in, final String token, final String force) throws Exception {
+        final String html = DOCTYPE_HTML
+            + "<html><head>\n"
+            + "<script>\n"
+            + LOG_TITLE_FUNCTION_NORMALIZE
+            + "  function test() {\n"
+            + "    var elem = document.getElementById('d1');\n"
+
+            + "    var config = { attributes: true, attributeOldValue: true };\n"
+            + "    var observer = new MutationObserver(function(mutations) {\n"
+            + "      mutations.forEach(function(mutation) {\n"
+            + "        log(mutation.attributeName + ' changed old: ' + mutation.oldValue);\n"
+            + "      });\n"
+            + "    });\n"
+            + "    observer.observe(elem, config);"
+
+            + "    var list = elem.classList;\n"
+            + "    if (!list) { log('no list'); return; }\n"
+
+            + "    log(elem.className);\n"
+            + "    log(list.length);\n"
+            + "    try {\n"
+            + "      var res = list.toggle('" + token + "', " + force + ");\n"
+            + "      log(res);\n"
+            + "    } catch(e) { logEx(e);}\n"
+            + "    log(list.length);\n"
+            + "    log(elem.className);\n"
+            + "    log(elem.outerHTML);\n"
+            + "  }\n"
+            + "</script></head>\n"
+            + "<body onload='test()'>\n"
+            + "  <div id='d1' class='" + in + "'></div>\n"
+            + "</body></html>";
+
+        loadPageVerifyTitle2(html);
+    }
+
     private void toggle(final String in, final String token) throws Exception {
         String html = DOCTYPE_HTML
             + "<html><head>\n"