Bug 1881457 - Keep saving mFocusedValue. r=smaug

This deletion was an oversight of the regressing bug. Kind of
embarrassing we didn't have a test for it...

Differential Revision: https://phabricator.services.mozilla.com/D202449

dom/html/HTMLTextAreaElement.cpp

@@ -481,7 +481,9 @@ nsresult HTMLTextAreaElement::PostHandleEvent(EventChainPostVisitor& aVisitor) {
   if (aVisitor.mEvent->mMessage == eFormSelect) {
     mHandlingSelect = false;
   }
-
+  if (aVisitor.mEvent->mMessage == eFocus) {
+    GetValueInternal(mFocusedValue, true);
+  }
   return NS_OK;
 }