RFR: 8255415: Nested calls to snap methods in Region give different results [v2]

[Johan Vos]

On Tue, 27 Oct 2020 10:38:34 GMT, Jose Pereda <jpereda 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.
>
> Jose Pereda has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update JBS reference in RegionTest

modules/javafx.graphics/src/main/java/javafx/scene/layout/Region.java line 303:

> 301:     }
> 302: 
> 303:     private static double scaledCeil(double value, double scale) {

Can you explicitly add here that this method guarantees 
`scaledCeil(scaledCeil(val, scale), scale) == scaledCeil (val, scale)` 

It seems other code already assumed this, but it should not be taken for granted, so now that your fix assures this is the case, it can be taken for granted.

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