<Swing Dev> RFR: 8249251: [dark_mode ubuntu 20.04] The selected menu is not highlighted in GTKLookAndFeel

[Pankaj Bansal]

Hi All,

Please review the following fix for jdk15.

Bug : https://bugs.openjdk.java.net/browse/JDK-8249251
webrev: http://cr.openjdk.java.net/~pbansal/8249251/webrev01/ 
<http://cr.openjdk.java.net/%7Epbansal/8249251/webrev01/>

Issue: In Ubuntu 20.04 in dark mode, the selected Menu is not being 
highlighted properly. so, there is no difference between selected and 
unselected Menu. The issue can be reproduced by running Swingset2 or 
using the test added in fix.

Cause: In dark mode, the highlight color for Menu is not visible over 
the dark background color for the Menubar. So, the highlight is not 
visible properly and it looks like there is no highlight being drawn.

Fix: The fix is to use some color for highlighting, which will be 
properly visible. We have taken the background color for selected text. 
This color is is visible over the dark Background easily. The fix is 
tested on Ubuntu 18.04, Ubuntu 20.04 and OL 8.2.

Added an automated test to verify that the highlight color is same as 
background color for selected text. The test passes on mach5 with 
multiple iterations. Link added in JBS.


Regards
Pankaj