Do you find the sliding that still happens annoying? I ask because I am considering making this "keep in same order" functionality optional in the future.

I think this feature is very important for comfortable browsing. Instead of sliding maybe it worth to think about initial order options for new tabs. 

I found, that tabs still sliding sometimes, however I cant say how it can be reproduced for now.

As I understand they must sliding in some cases and its not a bug?