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.

spec/system/page_objects/components/navigation_menu/base.rb

@@ -161,6 +161,8 @@ module PageObjects
             click_button(class: "sidebar-section-header-button", visible: false)
           end
 
+          expect(page).to have_css(".d-modal:not(.is-animating)")
+
           PageObjects::Modals::SidebarEditTags.new
         end