RFR: 8220484: JFXPanel renders a slanted image with a hidpi monitor scale of 125% or 175%

[Oliver Schmidtmer]

On Fri, 12 Jun 2020 20:43:18 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> @Schmidor in general, we don't use the `/issue` command to identify other issues that happen to be fixed by a commit.
>> Rather, if they are the same bug, or if one is caused by the other, we would closed the other one as a duplicate. This
>> seems like a good thing for me to add to the [CONTRIBUTING](https://github.com/openjdk/jfx/blob/master/CONTRIBUTING.md)
>> guidelines.
>
> Even though this is a seemingly simple fix, I'd like @prsadhuk to also review it.
> 
> @Schmidor Can you do the following?
> 
> 1. Provide an evaluation as to why this is the correct fix, and indicate what testing you have done to ensure no
> regressions in behavior. Changes to rounding in width and height computations tend to be trickier than they might seem.
> 2. Provide a test as part of this fix. Ideally, this would be an automated test, in the system tests project (under
> `tests/system`), and should set the `glass.win.uiScale` system property to `1.25` before running the test. There are a
> few examples you can follow under tests/system that create a JFrame with a JFXPanel and use Robot to verify the
> rendered image. If it turns out that an automated is not feasible, a manual test would be an acceptable alternative. 3.
> Enter the `/issue remove 8230007` command to remove that issue from this PR. I've already closed
> [JDK-8230007](https://bugs.openjdk.java.net/browse/JDK-8230007) in JBS as a duplicate.

I'll look into a more detailed description with code references and try to create a test.

The backbuffer of the Stage is created with Math.round(baseSize*uiScale) and for Swing the DataBuffer is referenced
into an BufferdImage, where the bounds are calculated with Math.ceil(baseSize*uiScale). So when the base-width is 101px
and uiscale is 1.25, the unrounded size is 126.25. The real buffer-width is then 126px, but rendered in swing using
127px. So there is an offset of one pixel per line and the resulting image is skewed.

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PR: https://git.openjdk.java.net/jfx/pull/246