RFR: 8231372: JFXPanel fails to render if setScene called on Swing thread

[Prasanta Sadhukhan]

On Fri, 4 Dec 2020 02:07:37 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> This fix was originally proposed by @mruzicka in PR #16 which was closed several months ago without being integrated. At the time we didn't have a test case that was failing.
> 
> While evaluating a bug that I filed, [JDK-8235843](https://bugs.openjdk.java.net/browse/JDK-8235843), I discovered that that bug was a duplicate of this one. The original proposed fix is correct, although I modified it slightly to add a try / finally so that the secondary event loop will be terminated even if the setScene throws an exception. I also added a unit test. Since the bulk of the fix is from PR #16, I will add the contributor of that PR.
> 
> NOTE to reviewers: I pushed two commits to my branch. The first is exactly the commit created for PR #16 and the second is the unit test along with the fix to the code to add try / finally. As always, they will be squashed into a single commit by Skara.

Marked as reviewed by psadhukhan (Reviewer).

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PR: https://git.openjdk.java.net/jfx/pull/362