With composer, it's just not possible to have a html toggle to switch between source and the text. Because text blocks are purely text nodes in the DOM and we cannot just toggle between html source and text unlike normal WYSIWYG editors.

When you compose in a WYSIWYG editor, you are practically generating HTML codes. Text blocks doesn't behave that way, they are purely text that can be manipulated. It is too heavy to render a WYSIWYG for each text block.

It is not just technically challenging but it's going to put your browser through a crazy stress test loading so many editor copies.