Pager links inside a modal do not open in the modal

[docwilmot]

Description of the bug

If a pager is rendered within a modal, the pager links do not contain relevant ajax attributes therefore breaking out of the modal when clicked.

Steps To Reproduce

• Create a page with a pager
• Open this page in a modal
• Click a pager link

Shortcut version if you dont want to create a pager page and a modal, replace the contents of one of the many layout modals with the output of a page with a pager:
I deleted everything in layout_block_configure_form() except return $form and replaced it with:

• module_load_include('inc', 'installer', 'installer.pages');
• $form['pager-page'] = installer_browser_page('module');

Then clicking any of the "Configure" dropbuttons on the layouck block page will pop up a modal with a pager.
Remember to reverse this.

Actual behavior

Clicking the pager links refreshes the whole page not the modal content

Expected behavior

The modal should load its content by ajax

Additional information

A View with a pager works fine in modals. Views uses theme_pager as well, but Views JS scans for pager links and adds the AJAX behavior
The following code snippet added to theme_pager_link() will fix this for backdrop modals (but not CKEditor modals):

  if (backdrop_is_dialog()) {
    $attributes += array(
      'class' => array('use-ajax'),
      'data-dialog' => TRUE,
    );
  }

However, CKEditor creates modals with ID #ckeditor5-modal, whereas our use-ajax magic link code is set to open a modal with ID #backdrop-modal. So if you add a non-Views pager to a CKEditor modal and click it, it doesnt re-use the modal but pops open a brand new modal on top of the existing CKEditor modal with a #backdrop-modal ID.