actually the previous fix was not fixing: now it does

Author: WebReflection

cjs/utils.js

@@ -2,8 +2,17 @@
 const Map = (require('@ungap/essential-map'));
 
 const append = (get, parent, children, start, end, before) => {
-  while (start < end)
-    parent.insertBefore(get(children[start++], 1), before);
+  const isSelect = 'selectedIndex' in parent;
+  let selectedIndex = -1;
+  while (start < end) {
+    const child = get(children[start], 1);
+    if (isSelect && selectedIndex < 0 && child.selected)
+      selectedIndex = start;
+    parent.insertBefore(child, before);
+    start++;
+  }
+  if (isSelect && -1 < selectedIndex)
+    parent.selectedIndex = selectedIndex;
 };
 exports.append = append;
 

esm/utils.js

@@ -1,8 +1,17 @@
 import Map from '@ungap/essential-map';
 
 export const append = (get, parent, children, start, end, before) => {
-  while (start < end)
-    parent.insertBefore(get(children[start++], 1), before);
+  const isSelect = 'selectedIndex' in parent;
+  let selectedIndex = -1;
+  while (start < end) {
+    const child = get(children[start], 1);
+    if (isSelect && selectedIndex < 0 && child.selected)
+      selectedIndex = start;
+    parent.insertBefore(child, before);
+    start++;
+  }
+  if (isSelect && -1 < selectedIndex)
+    parent.selectedIndex = selectedIndex;
 };
 
 export const eqeq = (a, b) => a == b;

index.js

@@ -39,9 +39,15 @@ try {
 var Map$1 = self.Map;
 
 var append = function append(get, parent, children, start, end, before) {
+  var isSelect = 'selectedIndex' in parent;
+  var selectedIndex = -1;
   while (start < end) {
-    parent.insertBefore(get(children[start++], 1), before);
+    var child = get(children[start], 1);
+    if (isSelect && selectedIndex < 0 && child.selected) selectedIndex = start;
+    parent.insertBefore(child, before);
+    start++;
   }
+  if (isSelect && -1 < selectedIndex) parent.selectedIndex = selectedIndex;
 };
 
 var eqeq = function eqeq(a, b) {

min.js

@@ -1218,6 +1218,26 @@ newState = domdiff(
 );
 console.timeEnd('reversed');
 
+document.body.selectedIndex = -1;
+newState = domdiff(
+  document.body,
+  newState,
+  []
+);
+
+var option = document.createTextNode('b');
+option.selected = true;
+newState = domdiff(
+  document.body,
+  newState,
+  [
+    document.createTextNode('a'),
+    option,
+    document.createTextNode('c')
+  ]
+);
+
+console.assert(document.body.selectedIndex === 1, 'selectedIndex is OK');
 // */
 
 tressa.end();