If you want to load Question Editor with CKEditor in a modal, your modal should not use a z-index higher than 10. Otherwise, modals used by CKEditor won't be visible anymore. CKEditor appends its modals to the end of the DOM, i.e. they do not have any parent element (other than the 'body' element). If you click on a CKEditor button to display a modal (e.g. the Math or Image modal), the modal is made visible with a z-index of 11 and moved into place. As the modal has no parent element, its place in the stacking order is calculated based on its z-index of 11. That means that it will be hidden behind other elements with a z-index higher than 11.
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