RFR: 8252935: Add treeShowing listener only when needed [v4]

[Jeanette Winzenburg]

On Tue, 9 Feb 2021 23:44:56 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> hmm ... might appear convenient (in very controlled contexts) but looks like a precondition violation: the sender of the change must not be null (concededly not explicitly spec'ed but logically implied, IMO)
>> 
>> so would tend to _not_ see this as blueprint for a general pattern fx code base
>
> I took a quick look at your latest change, and it seems reasonable to me now that you are calling a helper method rather than triggering a change method on the listener. I'll take a closer look later in the week. In the mean time @kleopatra and @arapte can provide their feedback.

not going for a full review - just a comment: agree with Kevin, the delegate methods are the way out, basically look good

`No further changes in testing required as it is all covered` - a bold statement .. looks like there's a missed null check (which was in the listener code but didn't make it into the delegate) at the end of sceneChanged

    windowChanged(oldScene.getWindow(), newScene.getWindow());

any of old/new scene can be null (or what am I missing?) If that's not covered by the tests .. ;)

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PR: https://git.openjdk.java.net/jfx/pull/185