feat(combobox): add on_scroll event to combobox

widget/src/combo_box.rs

@@ -67,7 +67,7 @@ use crate::core::{Element, Event, Length, Padding, Pixels, Rectangle, Shell, Siz
 use crate::overlay::menu;
 use crate::text::LineHeight;
 use crate::text_input::{self, TextInput};
-
+use crate::scrollable::Viewport;
 use std::cell::RefCell;
 use std::fmt::Display;
 
@@ -141,6 +141,7 @@ where
     on_open: Option<Message>,
     on_close: Option<Message>,
     on_input: Option<Box<dyn Fn(String) -> Message + 'a>>,
+    on_scroll: Option<Box<dyn Fn(Viewport) -> Message + 'a>>,
     padding: Padding,
     size: Option<f32>,
     shaping: text::Shaping,
@@ -180,6 +181,7 @@ where
             on_input: None,
             on_open: None,
             on_close: None,
+            on_scroll: None,
             padding: text_input::DEFAULT_PADDING,
             size: None,
             shaping: text::Shaping::default(),
@@ -217,6 +219,13 @@ where
         self
     }
 
+    /// Sets the message that will be produced when the menu of the
+    /// [`ComboBox`] is scrolled.
+    pub fn on_scroll(mut self, on_scroll: impl Fn(Viewport) -> Message + 'a) -> Self {
+        self.on_scroll = Some(Box::new(on_scroll));
+        self
+    }
+
     /// Sets the [`Padding`] of the [`ComboBox`].
     pub fn padding(mut self, padding: impl Into<Padding>) -> Self {
         self.padding = padding.into();
@@ -860,6 +869,10 @@ where
                 .shaping(self.shaping)
                 .ellipsis(self.ellipsis);
 
+                if let Some(on_scroll) = self.on_scroll.as_deref() {
+                    menu = menu.on_scroll(on_scroll);
+                }
+
                 if let Some(font) = self.font {
                     menu = menu.font(font);
                 }

widget/src/overlay/menu.rs

@@ -14,7 +14,7 @@ use crate::core::{
     Vector,
 };
 use crate::core::{Element, Shell, Widget};
-use crate::scrollable::{self, Scrollable};
+use crate::scrollable::{self, Scrollable, Viewport};
 
 /// A list of selectable options.
 pub struct Menu<'a, 'b, T, Message, Theme = crate::Theme, Renderer = crate::Renderer>
@@ -29,6 +29,7 @@ where
     to_string: &'a dyn Fn(&T) -> String,
     on_selected: Box<dyn FnMut(T) -> Message + 'a>,
     on_option_hovered: Option<&'a dyn Fn(T) -> Message>,
+    on_scroll: Option<&'a dyn Fn(Viewport) -> Message>,
     width: f32,
     padding: Padding,
     text_size: Option<Pixels>,
@@ -65,6 +66,7 @@ where
             to_string,
             on_selected: Box::new(on_selected),
             on_option_hovered,
+            on_scroll: None,
             width: 0.0,
             padding: Padding::ZERO,
             text_size: None,
@@ -112,6 +114,12 @@ where
         self
     }
 
+    /// Sets the message that will be produced when the [`Menu`] is scrolled.
+    pub fn on_scroll(mut self, on_scroll: &'a dyn Fn(Viewport) -> Message) -> Self {
+        self.on_scroll = Some(on_scroll);
+        self
+    }
+
     /// Sets the font of the [`Menu`].
     pub fn font(mut self, font: impl Into<Renderer::Font>) -> Self {
         self.font = Some(font.into());
@@ -200,6 +208,7 @@ where
             to_string,
             on_selected,
             on_option_hovered,
+            on_scroll,
             width,
             padding,
             font,
@@ -226,6 +235,12 @@ where
         })
         .height(menu_height);
 
+        let list = if let Some(on_scroll) = on_scroll {
+            list.on_scroll(on_scroll)
+        } else {
+            list
+        };
+
         state.tree.diff(&list as &dyn Widget<_, _, _>);
 
         Self {