FIX: keep details open in preview (#26518)

when morphing is enabled, details elements in the preview will be kept open

plugins/discourse-details/spec/system/preview_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+describe "Composer Preview", type: :system do
+  fab!(:user) { Fabricate(:user, refresh_auto_groups: true) }
+  let(:composer) { PageObjects::Components::Composer.new }
+
+  before { sign_in user }
+
+  it "keeps details element open when morphing content" do
+    SiteSetting.enable_diffhtml_preview = true
+
+    visit("/new-topic")
+
+    composer.type_content <<~MD
+      [details=Velcro]
+      What a rip-off!
+      [/details]
+    MD
+
+    within(composer.preview) do
+      find("details").click
+      expect(page).to have_css("details[open]")
+    end
+
+    composer.move_cursor_after("rip-off!")
+    composer.type_content(" :person_facepalming:")
+
+    within(composer.preview) { expect(page).to have_css("details[open] img.emoji") }
+  end
+end