RFR: 8324941: POC for Headless platform for JavaFX [v4]

[Andy Goryachev]

On Mon, 23 Jun 2025 08:26:30 GMT, Johan Vos <jvos at openjdk.org> wrote:

>> After spending a year in the sandbox repository, the Headless Platform is now ready to be reviewed in the main repository.
>> 
>> ### the Headless Platform
>> The Headless Platform is a top-level com.sun.glass.ui platform that replaces the second-level Monocle-Headless subplatform, that is part of the top-level Monocle platform. 
>> The platform can be used like any other platform, especially for running headless JavaFX applications, or for running tests (e.g. on CI systems)
>> 
>> ### changes
>> The code for the Headless Platform is in a new package com.sun.glass.ui.headless in the javafx.graphics module, and it does not require a code change in other packages.
>> This PR adds a simple change in the `build.gradle` file, to make the Headless Platform the standard when running headless tests (instead of using Monocle/Headless)
>> 
>> ### enable the Headless Platform
>> Setting the system property `glass.platform` to `Headless` will select the Headless Platform instead of the default one (either gtk, mac or win).
>> 
>> ### testing
>> `gradlew --info -PHEADLESS_TEST=true -PFULL_TEST=true :systemTests:cleanTest :systemTests:test`
>> runs all the system tests, apart from the robot tests. There are 2 failing tests, but there are valid reasons for those to fail.
>> 
>> ### robot tests
>> Most of the robot tests are working on headless as well. add `-PUSE_ROBOT` to test those.
>
> Johan Vos has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Process reviewer comments

WebView works:

![1751057115500](https://github.com/user-attachments/assets/15e8703f-93a0-4997-ae11-d3fd07fcc238)

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PR Comment: https://git.openjdk.org/jfx/pull/1836#issuecomment-3014314199