I really like this feature 
However, it...
However, it...
- will take time to implement (if I really do this, I want a fast "realtime" mirror).
- will be a feature for the advanced / expert user
- will need some tricks to allow the modification on both sides of the mirror (so you don't have to care which side is editable).
- will require CPU horsepower for the mirror transformation
- will support only one mirror per file and no mirror within subfiles (allowing more would be very hard to implement with performance in mind).
- will be necessary to introduce a new mirror mode and do some other fundamental changes to the code base (due to performance reasons)