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

[Jeanette Winzenburg]

On Fri, 23 Oct 2020 08:32:59 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.

modules/javafx.graphics/src/test/java/test/javafx/scene/layout/RegionTest.java line 1256:

> 1254: 
> 1255:     // Test for JDK-8199592
> 1256:     @Test public void snappingASnappedValueGivesTheSameValueTest() {

code comment still pointing to old issue :)

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