RFR: 8160755: bug6492108.java test fails with exception Image comparison failed at (0, 0) for image 4 in GTK L&F [v4]

[Abhishek Kumar]

> bug6492108.java test always fails in GTK L&F in single as well as dual screen linux machines. Since this test was not marked as "_headful_" in it's initial version, it never failed but after the fix of [JDK-8287051](https://bugs.openjdk.org/browse/JDK-8287051) this test was problem listed as it always
> failed which is captured in the JBS.
> The reason of failure is the pixel color mismatch between JEditorPane and JTextArea/JTextPane which is caused by the JEditorPane's default opaque value which is false for GTK3 at [L767](https://github.com/kumarabhi006/jdk/blob/c642f44bbe1e4cdbc23496a34ddaae30990ce7c0/src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/GTKStyle.java#L767).
> In initial load JEditorPane, JTextArea and JTextPane components are opaque at [L718](https://github.com/kumarabhi006/jdk/blame/6c7656678916ff3f5c9fc70efcbb69ce76801458/jdk/src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/GTKStyle.java#L718) but after the fix for [JDK-8145547](https://bugs.openjdk.org/browse/JDK-8145547) the implementation was changed to provide conditional support for GTK3 on linux where few components like Editor Pane, Formatted text Field, Password Field etc are opaque only if the  [GTK version is not 3](https://github.com/kumarabhi006/jdk/blame/73d2181d56063f6015e4fc42e130591bee39bc36/src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/GTKStyle.java#L746C21-L746C21).
> JTextPane's issue was observed by [JDK-8218479](https://bugs.openjdk.org/browse/JDK-8218479) and then the default opacity is set to true for JTextPane [irrespective of GTK version](https://github.com/kumarabhi006/jdk/blame/c642f44bbe1e4cdbc23496a34ddaae30990ce7c0/src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/GTKStyle.java#L750C16-L750C16).
> Extending the fix in isOpaque() method in GTKStyle.java for JEditorPane similar to JTextPane resolves the issue.
> 
> Test verified in both single and dual screen linux machines.

Abhishek Kumar has updated the pull request incrementally with one additional commit since the last revision:

  code formatting and jtreg skippedException added

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19381/files
  - new: https://git.openjdk.org/jdk/pull/19381/files/2d836f01..69fedf28

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19381&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19381&range=02-03

  Stats: 7 lines in 1 file changed: 2 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19381.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19381/head:pull/19381

PR: https://git.openjdk.org/jdk/pull/19381