Expose scrollable offset properly

core/src/widget/scrollable.rs

@@ -1,4 +1,4 @@
-use crate::{Align, Column, Justify, Length};
+use crate::{Align, Column, Justify, Length, Rectangle};
 
 #[derive(Debug)]
 pub struct Scrollable<'a, Element> {
@@ -112,11 +112,35 @@ impl<'a, Element> Scrollable<'a, Element> {
 
 #[derive(Debug, Clone, Copy, Default)]
 pub struct State {
-    pub offset: u32,
+    offset: u32,
 }
 
 impl State {
     pub fn new() -> Self {
         State::default()
     }
+
+    pub fn scroll(
+        &mut self,
+        delta_y: f32,
+        bounds: Rectangle,
+        content_bounds: Rectangle,
+    ) {
+        if bounds.height >= content_bounds.height {
+            return;
+        }
+
+        // TODO: Configurable speed (?)
+        self.offset = (self.offset as i32 - delta_y.round() as i32 * 15)
+            .max(0)
+            .min((content_bounds.height - bounds.height) as i32)
+            as u32;
+    }
+
+    pub fn offset(&self, bounds: Rectangle, content_bounds: Rectangle) -> u32 {
+        let hidden_content =
+            (content_bounds.height - bounds.height).round() as u32;
+
+        self.offset.min(hidden_content).max(0)
+    }
 }