Pressing return on additional participants page for event registration goes back to previous page instead of forward
There are three buttons on the additional participant page: Go Back, Continue, Skip Participant. Since all three are type="submit", when you press the return/enter key, the first button of type="submit" is clicked, so you go back one page. Contrast with the primary participant registration page, where return does take you to the next page because there is only one button or with a contribution page where return takes you to the confirmation page, again because there is only one button.
The easiest fix would be to change the types of the other buttons to "button". The button type is set here for all buttons as "submit" unless type="reset" is passed to addButtons. So I propose to review button types in use, setting those that make sense as "submit" (next, done, submit, upload, process, etc) and others as "button" (cancel, refresh). Any buttons without a type will be left as "submit" so as to avoid breaking anything.
There is also a class default added to buttons that have isDefault, but it's unclear to me what this does.