dnd_source: Add suppport for surface offset

The `drag_icon` callback is passed the offset of the cursor within the
widget at the start of the drag, and can return an offset the drag
surface should be placed relative to the cursor.

src/widget/dnd_source.rs

@@ -32,7 +32,7 @@ pub struct DndSource<'a, Message, D> {
     container: Element<'a, Message>,
     window: Option<window::Id>,
     drag_content: Option<Box<dyn Fn() -> D>>,
-    drag_icon: Option<Box<dyn Fn() -> (Element<'static, ()>, tree::State)>>,
+    drag_icon: Option<Box<dyn Fn(Vector) -> (Element<'static, ()>, tree::State, Vector)>>,
     on_start: Option<Message>,
     on_cancelled: Option<Message>,
     on_finish: Option<Message>,
@@ -90,7 +90,7 @@ impl<
     #[must_use]
     pub fn drag_icon(
         mut self,
-        f: impl Fn() -> (Element<'static, ()>, tree::State) + 'static,
+        f: impl Fn(Vector) -> (Element<'static, ()>, tree::State, Vector) + 'static,
     ) -> Self {
         self.drag_icon = Some(Box::new(f));
         self
@@ -102,7 +102,7 @@ impl<
         self
     }
 
-    pub fn start_dnd(&self, clipboard: &mut dyn Clipboard, bounds: Rectangle) {
+    pub fn start_dnd(&self, clipboard: &mut dyn Clipboard, bounds: Rectangle, offset: Vector) {
         let Some(content) = self.drag_content.as_ref().map(|f| f()) else {
             return;
         };
@@ -116,13 +116,14 @@ impl<
                 Some(iced_core::clipboard::DndSource::Widget(self.id.clone()))
             },
             self.drag_icon.as_ref().map(|f| {
-                let (icon, state) = f();
-                (
+                let (icon, state, offset) = f(offset);
+                iced_core::clipboard::IconSurface::new(
                     container(icon)
                         .width(Length::Fixed(bounds.width))
                         .height(Length::Fixed(bounds.height))
                         .into(),
                     state,
+                    offset,
                 )
             }),
             Box::new(content),
@@ -262,7 +263,11 @@ impl<
                                     if let Some(on_start) = self.on_start.as_ref() {
                                         shell.publish(on_start.clone())
                                     }
-                                    self.start_dnd(clipboard, state.cached_bounds);
+                                    let offset = Vector::new(
+                                        left_pressed_position.x - layout.bounds().x,
+                                        left_pressed_position.y - layout.bounds().y,
+                                    );
+                                    self.start_dnd(clipboard, state.cached_bounds, offset);
                                     state.is_dragging = true;
                                     state.left_pressed_position = None;
                                 }

src/widget/text_input/input.rs

@@ -1350,12 +1350,13 @@ where
                                     clipboard,
                                     false,
                                     id.map(|id| iced_core::clipboard::DndSource::Widget(id)),
-                                    Some((
+                                    Some(iced_core::clipboard::IconSurface::new(
                                         Element::from(
                                             TextInput::<'static, ()>::new("", input_text.clone())
                                                 .dnd_icon(true),
                                         ),
                                         iced_core::widget::tree::State::new(state_clone),
+                                        Vector::ZERO,
                                     )),
                                     Box::new(TextInputString(input_text)),
                                     DndAction::Move,