As a Chrome extension developer, I am developing an extension like 'Grammarly' and 'LanguageTool'. In a typical webpage rendered through DOM nodes, I can achieve the desired effect by utilizing the getClients() API along with a positioning and overlay strategy. However, this approach proves ineffective on Google Docs pages, given that Google Docs is currently rendered using canvas.(discussion about this).
Google App Script can interact with Google Docs programmatically. However, those extension I mentioned before seem that can work without it, just through local javaScript script. I'm not sure about 'LanguageTool', you can see it even can add a toolbar item.
Even with Google App Script, I only find an API to add menu item. I really want to know how do they do it.