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

[Kevin Rushforth]

On Mon, 27 Apr 2020 08:17:50 GMT, Arun Joseph <ajoseph at openjdk.org> wrote:

> After [JDK-8157900](https://bugs.openjdk.java.net/browse/JDK-8157900), initializeThickness() moved from
> ScrollBarThemeImpl to ScrollBarWidget but the implementation was kept the same by using testSBRef to call
> setThickness() for setting ScrollBarTheme.thickness.  Modifying initializeThickness() to set ScrollBarTheme.thickness
> once and removing testSBRef.

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.

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