RFR: 8255415: Nested calls to snap methods in Region give different results

[Jeanette Winzenburg]

On Tue, 27 Oct 2020 00:03:18 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> As discussed in the [JBS issue](https://bugs.openjdk.java.net/browse/JDK-8199592), when snapping an already snapped value (either intentionally or by  mistake), the result should be the same, otherwise we'll be jumping unnecessary from a valid pixel to another pixel.
>> 
>> This PR provides a fix to `snapSizeXX` methods used in `Region`, which ultimately use `Math.ceil`, by subtracting an epsilon value to scaled value before ceiling, to ensure snapping a snapped value gives the same value.
>> 
>> A test to verify `snapSizeX` and `snapSizeY` with 1000 random values, and 6 different UI scales is provided.
>> For the 1.0, 1.25, 1.5 and 2.0 UI scales, the current approach works fine. Only for 1.75 and the random 1.374562997 value fails (the test fails for around 2% of the values with 1.75 and around 10% with 1.374562997).
>> With the proposed fix, it doesn't fail at all.
>
> Looks good.
> 
> I can confirm that this fixes the problem of `snapSizeXY(snapSizeXY(val)) != snapSizeXY(val)` and that it fixes the test case from [JDK-8199592](https://bugs.openjdk.java.net/browse/JDK-8199592) when `sizeToScene` is called (as noted above this is a fix for one of the two underlying problems).
> 
> I also verified that the new test will catch the bug.

wondering about other places where scaled ceiling is used, f.i. snapPortion (which is package private and only used in HBox/VBox) - could they have the same problem?

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PR: https://git.openjdk.java.net/jfx/pull/336