RFR: 8171898: [WebView] ScrollBarWidget thickness calculation is not correct

[Arun Joseph]

On Tue, 28 Apr 2020 21:41:19 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> ScrollbarThemeJava delegates scrollbar thickness calculation to ScrollBarWidget::initializeThickness()
>> [ScrollBarWidget.java] method. Since "ScrollbarThemeJava::scrollbarThickness" is not associated with any
>> "ScrollBarWidget" instance, some workaround was made using special "ScrollBarWidget" instance named testRef.
>
> While this might be the right fix, I don't think the evaluation is right. In particular, this doesn't look related to
> [JDK-8157900](https://bugs.openjdk.java.net/browse/JDK-8157900). That refactoring fix was behavior neutral in terms of
> the object graph -- it uses an accessor pattern rather than the removed public `impl_` methods for encapsulation, but
> intentionally didn't change anything related to which instance of which objects are used. That fix was not backported
> to 8u and yet this bug is listed as affecting 8u.  So, what I'd like to see in the evaluation is a description of the
> root cause, and a brief explanation of the fix.  Can you also provide a test case that fails without the fix and passes
> with the fix? I don't see one in JBS that you can leverage, so you may need to create one.

This PR aims at modifying initializeThickness() to not use testSBRef and so, doesn't change the behavior of
ScrollBarWidget. As it's not a bug, test case can't be created.

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PR: https://git.openjdk.java.net/jfx/pull/197