blocks: adapt new row detection to block prefill changes (#46111)

wcs/blocks.py

@@ -349,4 +349,6 @@ class BlockWidget(WidgetList):
 
     def had_add_clicked(self):
         add_widget = self.get_widget('add_element')
-        return get_request().form.get(add_widget.name) if add_widget else False
+        request = get_request()
+        request_form = getattr(request, 'orig_form', request.form)
+        return request_form.get(add_widget.name) if add_widget else False

wcs/forms/root.py

@@ -519,7 +519,7 @@ class FormPage(Directory, FormTemplateMixin):
         had_prefill = False
         if page_change or submit_button is True:
             # on page change (or when a "add row" button is clicked), we
-            # fake a GET request so the form is not altered# with errors
+            # fake a GET request so the form is not altered with errors
             # from the previous submit; if the page was already
             # visited, we restore values; otherwise we set req.form as empty.
             req = get_request()
@@ -529,6 +529,11 @@ class FormPage(Directory, FormTemplateMixin):
                 data, form, displayed_fields, add_button_clicked=bool(submit_button is True)
             )
 
+            if submit_button is True:
+                # keep submitted data so it's possible to known the add button
+                # was clicked later on.
+                req.orig_form = req.form
+
             if had_prefill:
                 # include prefilled data
                 transient_formdata = self.get_transient_formdata()