Is it possible to show tooltip on a listbox header when mouse is hovered on top of the header.
Use a different way to display a header to your Listbox and then, there, you can display a tip on it.
As an example, you can add as needed Canvas, draw the Header texts, and in Mouse Enter (display a tip in a label at the window botto) / Mouse Exit (clear the Label contents).
Soetimes, been crative allows you to do what you want.
That way allows you to draw text as Bold, Italix, in colours, Aligned (centered, right aligned), set a different size, etc.
This is a custom Listbox Header based on a Canvas similar to what Emile was suggesting