Get CKEditor 5 to work in (modal) dialogs

Problem/Motivation

Similar to https://www.drupal.org/project/drupal/issues/3274937 🐛 Get CKEditor 5 to work in (modal) dialogs Fixed in core, when using ckeditor in an entity browser modal for editing inline content, you cannot use e.g. linkit because they use balloons that remain hidden under the modal.

Core uses in ckeditor5.dialog.fix.css:

.ui-dialog ~ .ck-body-wrapper {
  --ck-z-modal: 1261;
}

And ckeditor js hardcodes .ck.ck-balloon-panel to z-index: var(--ck-z-modal);. But as we aren't inside .ui-dialog, that's 1000, and the balloon is hidden.

Steps to reproduce

Proposed resolution

Add

<code>.ck.ck-balloon-panel {
  z-index: 1261 !important;
}

to entity_browser.modal.css.

Remaining tasks

User interface changes

API changes

Data model changes