RFR: 8264330: Scene MouseHandler is referencing removed nodes

[John Hendrikx]

On Wed, 31 Mar 2021 13:07:06 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> Small fix to clear a reference to a removed node left by Scene$MouseHandler.
>
> modules/javafx.graphics/src/test/java/test/javafx/scene/SceneTest.java line 1019:
> 
>> 1017:         pane = null;
>> 1018: 
>> 1019:         System.gc();
> 
> It is not sufficient to call `System.gc()` only once, since it will make for a fragile test. I recommend using the `JMemoryBuddy::assertCollectable` for this, which is what we are using for new tests.

This tool is in javafx.base's test folder, and it is not allowing me to reference it.  I'm not quite sure how to change the project to allow this.

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PR: https://git.openjdk.java.net/jfx/pull/448