I am planning to build a Google Workspace add-on. However, I am not fond of their CardService that isn't very flexible. I would like to make a more visually appealing interface of my own, using HTML, JS, CSS.
I know the Editor add-ons lets you do that, but it is the former framework and comes with many limitations, such as the single-app scope, the fact that they don't get shown as an icon on the "right-side panel", among other features.
Is there a way to inject HTML/JS content using the CardService to be able to build a custom interface for the sidebar?
It is not possible indeed: Google mentions that one shouldn't use HTML/CSS or client-side scripting
I thought it was possible since it is what I did unknowingly, and the interface worked great... until you have to reload the add-on, the homepageTrigger does not fire again when you close and re-open the add-on, and the user has an empty sidebar until they reload the entire page.