RFR: 8252935: Add treeShowing listener only when needed [v6]

[Kevin Rushforth]

On Tue, 16 Feb 2021 17:55:45 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add missed null checks to fix test failure
>
> modules/javafx.graphics/src/main/java/javafx/scene/SubScene.java line 2:
> 
>> 1: /*
>> 2:  * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
> 
> The initial year needs to be preserved. Please restore as follows: `2013, 2021, `

Alternatively, you can just restore the previous date range of `2013, 2018, ` and let our copyright update script handle bumping the "last modified year" the next time we run it.

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