When is an invention still "new" and patentable?

Technically, if a patent or printed publication discloses your invention, or if it has been in use or on sale, before you file a patent application, then you are prevented from getting a patent unless somehow the disclosure came from you as the inventor, either directly or indirectly, and also such disclosure had to be within one year of your patent application filing. If the disclosure was more than one year before your application filing, then such disclosure, even it was a disclosure from you as the inventor, prohibits you from obtaining a patent.

As for what is "public", and your communication with others in your field, that becomes a bit more complex. The basic/general rule would be that something is made public outside the group of inventors or the inventor's company to whom the invention would probably be assigned. However, and this is where the complexity comes into play, if you disclose the invention to a third party, that is, one of your "experts" within your noted circle, and then they suggest or implement an improvement or other embodiment, then they might be considered to be a co-inventor and therefore there has not really been a public disclosure beyond the inventorship entity.