The features request thread is locked or I'd put it there.
Every modern IDE in use today except for Adruino Web IDE displays all open files and allows for horizontal scrolling. This is an important feature because it allows moving from file to file with minimal effort in order to streamline development time.
I waste so much time selecting a file from the dropdown only to have it replace another file I was already working on that it's getting a little absurd. I like the Web IDE but it is essentially unusable for large projects because of this issue.
I work in Front End for a living, and could add this feature start to finish in less than 1 hour for free. Simply a matter of iterating through all tab items and then adding overflow scroll to the tab div. If I can help do so please let me know.