RFR: JDK-8016524: [macosx] Bottom line is not visible for JTableHeader [v4]

[Prasanta Sadhukhan]

On Mon, 31 Jan 2022 07:24:08 GMT, Prasanta Sadhukhan <psadhukhan at openjdk.org> wrote:

>> @prsadhuk I have changed the test case to explicitly set the table and header background to white. Mach5 tests are passing on all platforms including linux now. Can you please let me know in case you are seeing any issues on Ubuntu locally, with the new test case?
>
> I now see it fail locally in windows system also along with ubuntu. windows screenshot here where the border is white which is why it fails
> ![JTheaderBorder](https://user-images.githubusercontent.com/43534309/151753660-92d6021a-30d6-4215-97b0-7ab17ddb88d4.png)

It seems there's a anomaly for JTableHeader border in native and jdk in Windows L&F..
The system table shows here doesn't show any line
![JTheaderBorder-win](https://user-images.githubusercontent.com/43534309/151911992-846f2f21-c939-482f-9ef5-746d07c0b410.png)

whereas in our SwingSet2 we have a light grey line
![image](https://user-images.githubusercontent.com/43534309/151912084-c9cd39a5-74de-4ef9-a925-6081e8c4d294.png)

Technically, it might be construed as a bug but I guess I will prefer a demarcation line between header and body.
It's upto you if you want to take a look at this in a separate issue or in this.

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PR: https://git.openjdk.java.net/jdk/pull/7219