Combine main cart into newest

The last submitted patch, 2: combine_all_carts_in_the_newest-3404835-1.patch, failed testing. View results →
- codesniffer_fixes.patch Interdiff of automated coding standards fixes only.

I think this should be closed as duplicate of #3282729: Always use checkout cart as main →

Reading the description it will solve your issue as well and more robustly, since this could still fail since the sorting can change.
I'm going to mark this as postponed for now, feel free to put it back to needs work if you think it was not correct.

I'm going to do some work on the other issue.

Yeah, let's close this as a duplicate.