From #12857 (comment) (look for related issues and bug examples).

Looks like black background and global transparent panel hack used in some components from old days, maybe no needs in it now.

Search code by setBackground(new Color(0, 0, 0 and setExtBackgroundColor:

Must be tested in deck editor and game with active target/attack arrows with both java 8 and java 21 versions. Good result: colored and transparent backgrounds in card popup, chats, card panels, etc. Also must support main and battlefield background images (some themes use it).