RFR: 8252935: Add treeShowing listener only when needed

[John Hendrikx]

On Tue, 8 Sep 2020 20:16:20 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> @kevinrushforth 
>> 
>> I have another working alternative, but won't make a PR for it until it is more clear which direction the TableView / TreeTableView performance fixes are going to take.
>> 
>> The alternative would convert the `treeShowing` property in `Node` into a "lazy" property (something which JavaFX does not support directly at the moment).  A lazy property only binds to its dependencies when it is observed itself (so in this specific case, only when PopupWindow or ProgressIndicatorSkin are making use of it).
>> 
>> This means the property stays a part of `NodeHelper` but will only register its listeners on Window and Scene when it is observed itself.   Such lazy properties could be of great use in JavaFX in general, not just in this case.
>
> @hjohn Per [this message](https://mail.openjdk.java.net/pipermail/openjfx-dev/2020-September/027534.html) on the openjfx-dev mailing list, I have filed a new JBS issue for this PR to use. Please change the title to:
> 
> 8252935: Add treeShowing listener only when needed

So, will this actually get reviewed?

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