[28350] Fix bound element to pre block to avoid recursive insertion

When the outer pre element is not bound to the model code block, it is
not consumed for insertion and is thus treated as a container node where
we continue to insert _within_. This results in multiple code blocks
being nested into one another.

https://community.openproject.com/wp/28350

spec/features/wysiwyg/macros/code_block_macro_spec.rb

@@ -59,6 +59,37 @@ describe 'Wysiwyg code block macro', type: :feature, js: true do
         visit project_wiki_path(project, :wiki)
       end
 
+      it 'can add and save multiple code blocks (Regression #28350)' do
+
+        editor.in_editor do |container,|
+          editor.set_markdown expected
+
+          # Expect first macro saved to editor
+          expect(container).to have_selector('.op-ckeditor--code-block', text: snippet)
+          expect(container).to have_selector('.op-ckeditor--code-block-language', text: 'ruby')
+
+          editor.set_markdown "#{expected}\n#{expected}"
+          expect(container).to have_selector('.op-ckeditor--code-block', text: snippet, count: 2)
+          expect(container).to have_selector('.op-ckeditor--code-block-language', text: 'ruby', count: 2)
+        end
+
+        click_on 'Save'
+        expect(page).to have_selector('.flash.notice')
+
+        # Expect output widget
+        within('#content') do
+          expect(page).to have_selector('pre.highlight-ruby', count: 2)
+        end
+
+        # Edit page again, expect widget
+        click_on 'Edit'
+
+        editor.in_editor do |container,|
+          expect(container).to have_selector('.op-ckeditor--code-block', text: snippet, count: 2)
+          expect(container).to have_selector('.op-ckeditor--code-block-language', text: 'ruby', count: 2)
+        end
+      end
+
       it 'can add and edit a code block widget' do
         editor.in_editor do |container,|
           editor.click_toolbar_button 'Insert code snippet'