How to make slide-out control using fltk

Question

This is the layout

When user hover to blue/green group, it will expand itself by using resize, and if mouse leaving the group it will resize back to default size, I've tried using:

1. callback, immediately notice there's no event that handle enter/leave mouse.
2. overriding FL_GROUP and handle "handle(int e) manually, found out that FL_ENTER only got called once, FL_LEAVE only got called after mouse leave the window, instead of leaving the group.

Anymore idea?