RFR: 8242621: TabPane: Memory leak when switching skin [v4]

[Kevin Rushforth]

On Wed, 6 Jan 2021 11:16:07 GMT, Ambarish Rapte <arapte at openjdk.org> wrote:

>> `TabPaneSkin` installs some listeners that are not removed when `TabPaneSkin` is changed.
>> The fix converts listeners to WeakListeners and also removes them on dispose.
>> 
>> There is a NPE check change needed in `isHosrizontal()`. Without this check it causes NPE if pulse is in progress while TabPaneSkin is getting disposed.
>> 
>> `SkinMemoryLeakTest` already had a test which only needed to be enabled. 
>> Test fails before and passes after this change.
>
> Ambarish Rapte has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Space corrections in tests
>  - Review update: cleanup and corrections

Looks good.

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Marked as reviewed by kcr (Lead).

PR: https://git.openjdk.java.net/jfx/pull/318