RFR: 8349373: Support JavaFX preview features

[Michael Strauß]

On Tue, 4 Feb 2025 18:16:22 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> This PR contains a definition of preview features for JavaFX, as well as a helper class to verify that an application has opted into preview features.
>
> PREVIEW-FEATURES.md line 32:
> 
>> 30:    application has not opted into the use of preview features. All preview features have equal status
>> 31:    in any given JavaFX release and can not be enabled individually.
>> 32: 
> 
> do you want to talk about similarities and differences between this and jdk preview features https://openjdk.org/jeps/12 ?

This definition is, as you point out, largely lifted from JEP 12. Aside from wording changes, here are some key differences:

* JEP 12 assumes that a feature should be finished within two releases. I think for JavaFX, being just a framework and not a programming language, it is sensible to assume that we can finish features in the next release; that is, a preview feature that is introduced in JavaFX N should be finished in N+1.
* JEP 12 says that a preview feature should be at least 95% "done". I think we can be a little bit more risk-tolerant here, which is why I lowered this number to 90% "done".

The general idea is that preview features in JavaFX should be faster and accept a little bit more risk than preview features in the JDK.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1359#discussion_r1941716692