RFR: 8237469: CssStyleHelper reuse check fixed

[Dean Wookey]

On Fri, 17 Jan 2020 18:42:34 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> I mentioned this in the JBS bug as well: I suspect that [JDK-8234877](https://bugs.openjdk.java.net/browse/JDK-8234877) has the same root cause as this one. Can you run the HelloCSS test program as described in that bug with your patch from this PR and see if it also fixes that bug?
> 
> This will need (at least) two reviewers. I request @aghaisas and @dsgrieve to review (I'll review it as well next week, once I'm done with the more urgent reviews).

> I mentioned this in the JBS bug as well: I suspect that [JDK-8234877](https://bugs.openjdk.java.net/browse/JDK-8234877) has the same root cause as this one. Can you run the HelloCSS test program as described in that bug with your patch from this PR and see if it also fixes that bug?

I ran it and it seems to work. The button is always red in the red pane and always yellow in the yellow one.

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PR: https://git.openjdk.java.net/jfx/pull/87