PR 1936 [0] proposes to introduce observable attached properties that are distinguishable from instance properties with a naming convention (this is also relevant for PR 1714 [1]). However, before doing this, we should take a step back and see if we can't do better than that and make attached properties more accessible in the property API.
I've prepared a document that explores a few options for doing so, and a proposal for what I think might be a natural enhancement of the existing property metadata model [2]. Comments are welcome. [0] https://github.com/openjdk/jfx/pull/1936 [1] https://github.com/openjdk/jfx/pull/1714 [2] https://gist.github.com/mstr2/2fec0303fc440b8eaeb126befc76eb5c
