Example of Hover support for buttons, it basically requires handling the Event::Enter and Event::Leave events.

hover_button.rs

use fltk::{
    app, button,
    enums::{Color, Event, FrameType},
    prelude::*,
    window,
};

const COLOR: u32 = 0xB39DDB;
const SELECTION_COLOR: u32 = 0x9575CD;
const HOVER_COLOR: u32 = 0xD1C4E9;

fn main() {
    let app = app::App::default();
    app::set_visible_focus(false);
    let mut win = window::Window::default().with_size(400, 300);
    win.set_color(Color::White);
    let mut but = button::Button::default()
        .with_size(80, 40)
        .with_label("Click!")
        .center_of_parent();
    but.set_color(Color::from_hex(COLOR));
    but.set_selection_color(Color::from_hex(SELECTION_COLOR));
    but.set_frame(FrameType::FlatBox);
    win.end();
    win.show();
    but.handle(move |b, ev| match ev {
        Event::Enter => {
            b.set_color(Color::from_hex(HOVER_COLOR));
            b.redraw();
            true
        }
        Event::Leave => {
            b.set_color(Color::from_hex(COLOR));
            b.redraw();
            true
        }
        _ => false,
    });
    but.set_callback(|_| println!("Clicked!"));
    app.run().unwrap();
}