RFR: 8251946: ObservableList.setAll does not conform to specification

[Kevin Rushforth]

On Tue, 18 Aug 2020 19:50:55 GMT, Leon Linhart <github.com+4029915+TheMrMilchmann at openjdk.org> wrote:

> Hi, this PR fixes [JDK-8251946](https://bugs.openjdk.java.net/browse/JDK-8251946) computing whether the list was
> actually modified instead of just returning `true`. The list was modified if 1. it was not empty (modified by calling
> `#clear()`), or if 2. it was modified as result of the `#addAll()` call.
> 
> If you want any test coverage for this please let me know.
> 
> I reported the issue a couple of days ago via web formula and waited for the confirmation to open this PR but now I see
> that @kevinrushforth (sorry for pinging) is already assigned to the JBS issue. I'm not too familiar with how you handle
> JBS issues or more specifically whether assigning is used to indicate that the issue is in the assignee's "domain", or
> that the assignee is already working on it. In the latter case, please feel free to close this PR. (My OCA submission
> is still pending anyway.)

@TheMrMilchmann I am not actively working on that bug, so you can proceed with this PR. I will review it when it is
ready.

Once you have submitted the OCA, please add a comment to this PR with the `/signed` command (and nothing else in the
comment). After your OCA is approved and recorded, the Skara bot will mark it as ready for review.

Please do add a unit test for this. You can find existing collections tests
[here](https://github.com/openjdk/jfx/tree/master/modules/javafx.base/src/test/java/test/javafx/collections).

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PR: https://git.openjdk.java.net/jfx/pull/284