Swing: Floating panel next to the selected item in a JComboBox

Answer 1

It's not possible with the custom renderer since Swing components are light weight. That is, Java is given a native window and all the component drawing takes place in that window. In your case, that is the JFrame containing the combo box.

What you can do though is create a new undecorated window and set it's location accordingly and draw whatever you want inside it.

EDIT: When Java needs to paint outside it's window bounds (like the case of pop up messages or combo boxes drop downs) if the component falls inside the bounds it uses the swing light weight mechanism. But if the component falls out side the bounds it is automatically substituted with a awt heavy weight component that has it's own native drawing surface outside the active window.

Answer 2

I've implemented similar idea using combobox renderers and tooltips on them. Content of every item's tooltip can be customized and rendered using HTML. Location of the tooltip can be set outside of the item itself thus creating design very similar to the one presented in your question.

Here is the starting point for you:

http://www.java2s.com/Code/Java/Swing-Components/ToolTipComboBoxExample.htm