Update selectedcontent in selectedness setting

This PR makes sure that the contents of the selectedcontent element stay
up to date when the selected option is changed in the selectedness
setting algorithm.

This issue was found here:
web-platform-tests/wpt#55849 (comment)

Author: josepharhar

source

@@ -56241,24 +56241,45 @@ interface <dfn interface>HTMLSelectElement</dfn> : <span>HTMLElement</span> {
   <var>element</var>, is to run the following steps:</p>
 
   <ol>
-   <li><p>If <var>element</var>'s <code data-x="attr-select-multiple">multiple</code> attribute is
-   absent, and <var>element</var>'s <span data-x="concept-select-size">display size</span> is 1,
-   and no <code>option</code> elements in the <var>element</var>'s <span
-   data-x="concept-select-option-list">list of options</span> have their <span
-   data-x="concept-option-selectedness">selectedness</span> set to true, then set the <span
-   data-x="concept-option-selectedness">selectedness</span> of the first <code>option</code>
-   element in the <span data-x="concept-select-option-list">list of options</span> in
-   <span>tree order</span> that is not <span data-x="concept-option-disabled">disabled</span>,
-   if any, to true, and return.</p></li>
-
-   <li><p>If <var>element</var>'s <code data-x="attr-select-multiple">multiple</code> attribute is
-   absent, and two or more <code>option</code> elements in <var>element</var>'s <span
-   data-x="concept-select-option-list">list of options</span> have their <span
-   data-x="concept-option-selectedness">selectedness</span> set to true, then set the <span
-   data-x="concept-option-selectedness">selectedness</span> of all but the last <code>option</code>
-   element with its <span data-x="concept-option-selectedness">selectedness</span> set to true in
-   the <span data-x="concept-select-option-list">list of options</span> in <span>tree order</span>
-   to false.</p></li>
+   <li><p>Let <var>updateSelectedcontent</var> be false.</p></li>
+
+   <li>
+    <p>If <var>element</var>'s <code data-x="attr-select-multiple">multiple</code> attribute is
+    absent, and <var>element</var>'s <span data-x="concept-select-size">display size</span> is 1,
+    and no <code>option</code> elements in the <var>element</var>'s <span
+    data-x="concept-select-option-list">list of options</span> have their <span
+    data-x="concept-option-selectedness">selectedness</span> set to true, then:</p>
+
+    <ol>
+     <li><p>Set the <span
+     data-x="concept-option-selectedness">selectedness</span> of the first <code>option</code>
+     element in the <span data-x="concept-select-option-list">list of options</span> in
+     <span>tree order</span> that is not <span data-x="concept-option-disabled">disabled</span>,
+     if any, to true.</p></li>
+
+     <li><p>Set <var>updateSelectedcontent</var> to true.</p></li>
+    </ol>
+   </li>
+
+   <li>
+    <p>Otherwise, if <var>element</var>'s <code data-x="attr-select-multiple">multiple</code>
+    attribute is absent, and two or more <code>option</code> elements in <var>element</var>'s <span
+    data-x="concept-select-option-list">list of options</span> have their <span
+    data-x="concept-option-selectedness">selectedness</span> set to true, then:</p>
+
+    <ol>
+     <li><p>Set the <span data-x="concept-option-selectedness">selectedness</span> of all but the
+     last <code>option</code> element with its <span
+     data-x="concept-option-selectedness">selectedness</span> set to true in the <span
+     data-x="concept-select-option-list">list of options</span> in <span>tree order</span> to
+     false.</p></li>
+
+     <li><p>Set <var>updateSelectedcontent</var> to true.</p></li>
+    </ol>
+   </li>
+
+   <li><p>If <var>updateSelectedcontent</var> is true, then run <span>update a <code>select</code>'s
+   <code>selectedcontent</code></span> given <var>element</var>.</p></li>
   </ol>
   </div>