RFR: 8236753: Animations do not play backwards after being stopped

[Kevin Rushforth]

On Wed, 8 Jan 2020 23:52:07 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

> The private field `lastPlayFinished` is responsible for 2 cases where an animation in `STOPPED` status does not play after `play()` is called if the rate is negative:
> 
> 1. When the animation is created, it is `STOPPED` and `lastPlayFinished` is `false`. Setting a negative rate and calling `play()` will not jump to the end of the animation (in order to play it backwards) because the `if (lastPlayedFinished)` check is `false`. Creating the animation with `lastPlayFinished = true` fixes this. However, `SequentialTransitionPlayTest#testCycleReverse`'s initial state test implies that the original behavior is correct. *That test currently fails with this change.* Either the fix is reverted or the test is corrected.
> 2. When the animation is stopped (if it was not `STOPPED` already), `jumpTo(Duration.ZERO)` sets `lastPlayFinished` to `false`, which causes the same issue above with `play()`. Setting `lastPlayFinished = true` at the end `stop()` fixes this issue.
> 
> A test was added for case 2 to check that the playing head indeed jumps to the end of the animation. Without this fix, it stays at the start.
> 
> I'm still somewhat confused as to what constitutes a "last play finished". Any `jumpTo` resets `lastPlayFinished` to `false`, even if the jump is to the start/end of the animation. In this case, stopping an animation, jumping to its start/end, setting the rate to negative/positive, and playing, will do nothing as the end condition is reached immediately. This is what the behavior that was fixed for cases 1 and 2, but maybe this is also incorrect behavior for jumping to start/end.
> 
> A test app is included in the "parent" [bug](https://bugs.openjdk.java.net/browse/JDK-8210238), which also mentions a bug relating to **pausing** and playing backwards, so be mindful of it when testing.

I'll review this next week. This seems a fine candidate for openjfx14, so it (along with a couple other pending reviews that can be for 14) will be a good test of targeting a PR to the stabilization branch.

I also request @arapte to review.

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PR: https://git.openjdk.java.net/jfx/pull/82