There seems to be an issue with fabric ui surfaces. I am trying to incorporate tinyMCE into a pcf control. As shown in the screen shot, the surface doesn't seem to properly encapsulate tinyMCE. The format menu in the surface is showing up behind the surface.
Any reason why you want to use TinyMCE?
A few people have got quilljs to work with react well https://pcf.gallery/d365richtexteditor/ is a good example to work from.
Can you please check the CSS , looks like Z-index for the flyout need to be adjusted. There is a community tinyMCE control which can be referenced - https://pcf.gallery/tinymce-rich-text-editor/
The rich text project doesn't encapsulate tinyMCE within a fabric-ui modal surface. Other 3rd party editor projects seem to have the same issue.
Also the documentation doesn't indicate that you can control the z-index.