In Twitter's web app, after the tweet body in Compose Tweet, the next tabindex is the Add emoji button. However the primary action is the Tweet button.
In GitHub's web app, after the comment body in Leave a Comment on a Pull Request, the next tabindex is the Select a File to Upload link. However the primary action is the Comment button (which had tabindex priority for years, and was only changed recently).
A GitHub support agent told me that it is to do with accessibility.
What is the accessibility best practice on tabindex here? (I am surprised that GitHub and Twitter don't have a solution that is both good for accessibility but still allows the next tabindex to be the primary action)