<Swing Dev> [12] RFR JDK-8218479: JTextPane display issue with GTKLookAndFeel

[Semyon Sadetsky]

Hello,

Please review a fix for JDK 12.

JBS: https://bugs.openjdk.java.net/browse/JDK-8218479

webrev: http://cr.openjdk.java.net/~ssadetsky/8218479/webrev.00/

This is a JCK test failure fix. The test fails under the GTKLookAndFeel 
on Linux platforms with gtk+-3.20+ library installed.

Starting gtklib- 3.20 the  widget drawing styles were totally reworked 
so the background layer of the TEXT_PANE widget should be painted 
explicitly. To fix the issue the corresponding area type is made opaque. 
The fix was tested on all major supported Linux platforms.

--Semyon