That custom Swing layout manager is a standard Java layout manager since Java 6, released in 2006.
https://docs.oracle.com/javase/6/docs/api/javax/swing/GroupL...
Sun prototyped it on Netbeans, made it open source and then when everyone was happy, it became yet another layout manager available in any Java compliant platform.
That is the whole point of a layout manager engine, they are extensible.
Which is why project Houdini is having a layout engine APIs as well.
https://developers.google.com/web/updates/2016/05/houdini#la...
Matisse is the only UI designer that I know that does true drag and drop (letting you position widgets exactly where you want them) while also producing flexible layout. And IIRC they had to write a custom Swing layout manager for that.