FlatLaf compatibility

[illumi]

FlatLaf is becoming common in Java Swing applications as a direct plug-in for "modern" themes. Unfortunately the interaction between JTreeTable and FlatLaf seems to create some UI drawing errors in cells and tree expansion.

To reproduce, add the FlatLaf dependency, call FlatLightLaf.setup(); or FlatDarkLaf.setup(); to initialise the Swing look and feel and try to navigate a JTreeTable component with multiple rows and columns.

[javagl]

From a quick test, I noticed two main issues:

• The drawing errors: It is leaving blue "bars" at the border when selecting other rows (in the beginning of the GIF)
• The expansion issue: It does not expand/collapse the path that is double-clicked on (at the end of the GIF)

I see that this is an issue, but

• The source code was mainly taken from the article that is mentioned in the README and LICENSE
• I'll probably not have time to investigate this in the near term. (There's always the option to pay me to do so, but without investigating it further, it's hard to promise that I could solve it "quickly", and ... nobody is paying anybody anyhow...)

[illumi]

Thanks for the quick feedback! Yes, those are also the bugs I have experienced. There's no expectation of course, I am just reporting what I have discovered :)