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

[John Hendrikx]

On Fri, 22 Jan 2021 21:36:06 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> John Hendrikx has updated the pull request incrementally with four additional commits since the last revision:
>> 
>>  - Add missing TreeShowingExpressionTest which also tests SubScene
>>  - Also dispose listeners on Window/Showing in dispose
>>  - Fix review comments
>>  - Update copyrights
>
> modules/javafx.graphics/src/main/java/javafx/scene/SubScene.java line 312:
> 
>> 310:                     _value.setTreeVisible(isTreeVisible());
>> 311:                     _value.setDisabled(isDisabled());
>> 312:                     _value.setTreeShowing(isTreeShowing());
> 
> This looks fine. It might be worth adding a unit test to ensure that `isTreeShowing` works correctly for nodes in a `SubScene`.

I've added a test for this in `TreeShowingExpressionTest`.

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