UI: better modal backdrop opacity fading (#26328)

The fading should now be function of the swipe position. We should also correctly instantly remove the fading when closing the modal.

plugins/chat/spec/system/page_objects/chat/chat_channel.rb

@@ -86,6 +86,7 @@ module PageObjects
       def bookmark_message(message)
         if page.has_css?("html.mobile-view", wait: 0)
           click_message_action_mobile(message, "bookmark")
+          expect(page).to have_css(".d-modal:not(.is-animating)")
         else
           hover_message(message)
           find(".bookmark-btn").click