diff --git a/examples/custom-widgets.rs b/examples/custom-widgets.rs
new file mode 100644
index 0000000..6137a8d
--- /dev/null
+++ b/examples/custom-widgets.rs
@@ -0,0 +1,120 @@
+//! This example shows two approaches to writing custom widgets: implementing
+//! traits or using the [`Custom`] widget with callbacks.
+
+use gooey::value::Dynamic;
+use gooey::widget::{MakeWidget, Widget, HANDLED};
+use gooey::widgets::Custom;
+use gooey::Run;
+use kludgine::figures::units::{Lp, UPx};
+use kludgine::figures::{ScreenScale, Size};
+use kludgine::Color;
+
+fn main() -> gooey::Result {
+    "Inline Widgets"
+        .and(callback_widget())
+        .into_rows()
+        .and(
+            "impl MakeWidget"
+                .and(ToggleMakeWidget::default())
+                .into_rows(),
+        )
+        .and("impl Widget".and(impl_widget()).into_rows())
+        .into_columns()
+        .centered()
+        .expand()
+        .run()
+}
+
+/// This function returns a [`Custom`] widget that implements its functionality
+/// using callbacks.
+///
+/// This approach was added to make it easy to create one-off widgets in a
+/// hierarchy to handle events or other purpose-built functions.
+fn callback_widget() -> impl MakeWidget {
+    // This implementation and the impl `Widget` implementation both use the
+    // same Dynamic value setup.
+    let toggle = Toggle::default();
+
+    Custom::empty()
+        .background_color(toggle.color)
+        .on_hit_test(|_, _| true)
+        .on_mouse_down(move |_, _, _, _| {
+            toggle.value.toggle();
+            HANDLED
+        })
+        .height(Lp::inches(1))
+}
+
+/// A second approach is to implement [`MakeWidget`] for a type. This allows any
+/// type to be used when composing your UI that know how to create a widget.
+///
+/// This enables using callback-based widgets (or any other combination of
+/// widgets) in a reusable fashion.
+#[derive(Default)]
+struct ToggleMakeWidget(Toggle);
+
+impl MakeWidget for ToggleMakeWidget {
+    fn make_widget(self) -> gooey::widget::WidgetInstance {
+        // In a real code base, the contents of callback_widget() would go here
+        callback_widget().make_widget()
+    }
+}
+
+/// This function returns [`Toggle`] using its [`Widget`] implementation.
+///
+/// This is the lowest-level way to implement a Widget, but it also provides the
+/// most power and flexibility.
+fn impl_widget() -> impl MakeWidget {
+    Toggle::default()
+}
+
+#[derive(Debug)]
+struct Toggle {
+    value: Dynamic<bool>,
+    color: Dynamic<Color>,
+}
+
+impl Default for Toggle {
+    fn default() -> Self {
+        let value = Dynamic::default();
+        let color = value.map_each(|on| if *on { Color::RED } else { Color::BLUE });
+        Self { value, color }
+    }
+}
+
+impl Widget for Toggle {
+    fn redraw(&mut self, context: &mut gooey::context::GraphicsContext<'_, '_, '_, '_, '_>) {
+        context.fill(self.color.get_tracking_refresh(context));
+    }
+
+    fn layout(
+        &mut self,
+        available_space: Size<gooey::ConstraintLimit>,
+        context: &mut gooey::context::LayoutContext<'_, '_, '_, '_, '_>,
+    ) -> Size<UPx> {
+        Size::new(
+            available_space.width.min(),
+            Lp::inches(1).into_upx(context.gfx.scale()),
+        )
+    }
+
+    fn hit_test(
+        &mut self,
+        _location: kludgine::figures::Point<kludgine::figures::units::Px>,
+        _context: &mut gooey::context::EventContext<'_, '_>,
+    ) -> bool {
+        true
+    }
+
+    fn mouse_down(
+        &mut self,
+        _location: kludgine::figures::Point<kludgine::figures::units::Px>,
+        _device_id: kludgine::app::winit::event::DeviceId,
+        _button: kludgine::app::winit::event::MouseButton,
+        _context: &mut gooey::context::EventContext<'_, '_>,
+    ) -> gooey::widget::EventHandling {
+        self.value.toggle();
+
+        HANDLED
+    }
+}
diff --git a/src/context.rs b/src/context.rs
index 87f0d59..c1a672c 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -10,7 +10,7 @@ use kludgine::app::winit::event::{
     DeviceId, Ime, KeyEvent, MouseButton, MouseScrollDelta, TouchPhase,
 };
 use kludgine::figures::units::{Lp, Px, UPx};
-use kludgine::figures::{IntoSigned, Point, Px2D, Rect, ScreenScale, Size, Zero};
+use kludgine::figures::{IntoSigned, Point, Px2D, Rect, Round, ScreenScale, Size, Zero};
 use kludgine::shapes::{Shape, StrokeOptions};
 use kludgine::{Color, Kludgine};
 
@@ -610,7 +610,7 @@ impl<'context, 'window, 'clip, 'gfx, 'pass> GraphicsContext<'context, 'window, '
         );
 
         let background = self.get(&WidgetBackground);
-        self.gfx.fill(background);
+        self.fill(background);
 
         self.apply_current_font_settings();
 
@@ -707,7 +707,8 @@ impl<'context, 'window, 'clip, 'gfx, 'pass> LayoutContext<'context, 'window, 'cl
             .clone()
             .lock()
             .as_widget()
-            .layout(available_space, self);
+            .layout(available_space, self)
+            .map(Round::ceil);
         if self.persist_layout {
             self.graphics
                 .current_node
diff --git a/src/lib.rs b/src/lib.rs
index 8a4c6c4..a4c81e8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -42,6 +42,16 @@ pub enum ConstraintLimit {
 }
 
 impl ConstraintLimit {
+    /// Returns `UPx::ZERO` when sizing to fit, otherwise it returns the size
+    /// being filled.
+    #[must_use]
+    pub fn min(self) -> UPx {
+        match self {
+            ConstraintLimit::Fill(v) => v,
+            ConstraintLimit::SizeToFit(_) => UPx::ZERO,
+        }
+    }
+
     /// Returns the maximum measurement that will fit the constraint.
     #[must_use]
     pub fn max(self) -> UPx {
diff --git a/src/styles/components.rs b/src/styles/components.rs
index eb69642..13d6968 100644
--- a/src/styles/components.rs
+++ b/src/styles/components.rs
@@ -20,6 +20,7 @@ use crate::styles::{Dimension, FocusableWidgets, FontFamilyList, VisualOrder};
 /// use gooey::styles::Dimension;
 /// use gooey::styles::components::{SurfaceColor, TextColor};
 /// use gooey::kludgine::Color;
+/// use gooey::kludgine::figures::Zero;
 ///
 /// define_components! {
 ///     GroupName {
diff --git a/src/tree.rs b/src/tree.rs
index afdf43f..5b8212d 100644
--- a/src/tree.rs
+++ b/src/tree.rs
@@ -132,8 +132,8 @@ impl Tree {
 
         let node = &mut data.nodes[parent];
         if let Some(cached_layout) = &node.last_layout_query {
-            if constraints.width.max() < cached_layout.constraints.width.max()
-                && constraints.height.max() < cached_layout.constraints.height.max()
+            if constraints.width.max() <= cached_layout.constraints.width.max()
+                && constraints.height.max() <= cached_layout.constraints.height.max()
             {
                 return Some(cached_layout.size);
             }
diff --git a/src/value.rs b/src/value.rs
index 52bfe47..4d81840 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -3,7 +3,7 @@
 use std::cell::Cell;
 use std::fmt::{Debug, Display};
 use std::future::Future;
-use std::ops::{Deref, DerefMut};
+use std::ops::{Deref, DerefMut, Not};
 use std::str::FromStr;
 use std::sync::{Arc, Mutex, MutexGuard, TryLockError};
 use std::task::{Poll, Waker};
@@ -134,6 +134,19 @@ impl<T> Dynamic<T> {
         self.0.map_mut(|value, _| map(value)).expect("deadlocked")
     }
 
+    /// Updates the value to the result of invoking [`Not`] on the current
+    /// value. This function returns the new value.
+    #[allow(clippy::must_use_candidate)]
+    pub fn toggle(&self) -> T
+    where
+        T: Not<Output = T> + Clone,
+    {
+        self.map_mut(|value| {
+            *value = !value.clone();
+            value.clone()
+        })
+    }
+
     /// Returns a new dynamic that is updated using `U::from(T.clone())` each
     /// time `self` is updated.
     #[must_use]
diff --git a/src/widget.rs b/src/widget.rs
index 13a982c..5c74c6b 100644
--- a/src/widget.rs
+++ b/src/widget.rs
@@ -42,11 +42,14 @@ pub trait Widget: Send + UnwindSafe + Debug + 'static {
 
     /// Layout this widget and returns the ideal size based on its contents and
     /// the `available_space`.
+    #[allow(unused_variables)]
     fn layout(
         &mut self,
         available_space: Size<ConstraintLimit>,
         context: &mut LayoutContext<'_, '_, '_, '_, '_>,
-    ) -> Size<UPx>;
+    ) -> Size<UPx> {
+        available_space.map(ConstraintLimit::min)
+    }
 
     /// Return true if this widget should expand to fill the window when it is
     /// the root widget.
@@ -250,7 +253,7 @@ pub trait WrapperWidget: Debug + Send + UnwindSafe + 'static {
         available_space: Size<ConstraintLimit>,
         context: &mut LayoutContext<'_, '_, '_, '_, '_>,
     ) -> WrappedLayout {
-        let adjusted_space = self.adjust_child_constraint(available_space, context);
+        let adjusted_space = self.adjust_child_constraints(available_space, context);
         let child = self.child_mut().mounted(&mut context.as_event_context());
         let size = context
             .for_other(&child)
@@ -263,7 +266,7 @@ pub trait WrapperWidget: Debug + Send + UnwindSafe + 'static {
     /// Returns the adjusted contraints to use when laying out the child.
     #[allow(unused_variables)]
     #[must_use]
-    fn adjust_child_constraint(
+    fn adjust_child_constraints(
         &mut self,
         available_space: Size<ConstraintLimit>,
         context: &mut LayoutContext<'_, '_, '_, '_, '_>,
@@ -430,7 +433,7 @@ where
     fn redraw(&mut self, context: &mut GraphicsContext<'_, '_, '_, '_, '_>) {
         let background_color = self.background_color(context);
         if let Some(color) = background_color {
-            context.gfx.fill(color);
+            context.fill(color);
         }
 
         self.redraw_background(context);
diff --git a/src/widgets.rs b/src/widgets.rs
index 2bd7c4d..079bbcf 100644
--- a/src/widgets.rs
+++ b/src/widgets.rs
@@ -5,6 +5,7 @@ pub mod button;
 mod canvas;
 pub mod checkbox;
 pub mod container;
+mod custom;
 mod expand;
 pub mod input;
 pub mod label;
@@ -24,6 +25,7 @@ pub use button::Button;
 pub use canvas::Canvas;
 pub use checkbox::Checkbox;
 pub use container::Container;
+pub use custom::Custom;
 pub use expand::Expand;
 pub use input::Input;
 pub use label::Label;
diff --git a/src/widgets/container.rs b/src/widgets/container.rs
index 7b28c4d..27ea465 100644
--- a/src/widgets/container.rs
+++ b/src/widgets/container.rs
@@ -188,7 +188,7 @@ impl WrapperWidget for Container {
         })
     }
 
-    fn adjust_child_constraint(
+    fn adjust_child_constraints(
         &mut self,
         available_space: Size<ConstraintLimit>,
         context: &mut LayoutContext<'_, '_, '_, '_, '_>,
diff --git a/src/widgets/custom.rs b/src/widgets/custom.rs
new file mode 100644
index 0000000..baf5057
--- /dev/null
+++ b/src/widgets/custom.rs
@@ -0,0 +1,779 @@
+use std::fmt::Debug;
+use std::panic::UnwindSafe;
+
+use kludgine::app::winit::event::{
+    DeviceId, Ime, KeyEvent, MouseButton, MouseScrollDelta, TouchPhase,
+};
+use kludgine::figures::units::Px;
+use kludgine::figures::{Point, Size};
+use kludgine::Color;
+
+use crate::context::{EventContext, GraphicsContext, LayoutContext, WidgetContext};
+use crate::value::{IntoValue, Value};
+use crate::widget::{EventHandling, MakeWidget, WidgetRef, WrappedLayout, WrapperWidget, IGNORED};
+use crate::widgets::Space;
+use crate::ConstraintLimit;
+
+/// A callback-based custom widget.
+///
+/// This type can be used to create inline widgets without defining a new type
+/// and implementing [`Widget`]/[`WrapperWidget`] for it.
+#[must_use]
+pub struct Custom {
+    child: WidgetRef,
+    redraw_foreground: Option<Box<dyn RedrawFunc>>,
+    redraw_background: Option<Box<dyn RedrawFunc>>,
+    mounted: Option<Box<dyn EventFunc>>,
+    unmounted: Option<Box<dyn EventFunc>>,
+    background: Option<Value<Color>>,
+    unhover: Option<Box<dyn EventFunc>>,
+    focus: Option<Box<dyn EventFunc>>,
+    blur: Option<Box<dyn EventFunc>>,
+    activate: Option<Box<dyn EventFunc>>,
+    deactivate: Option<Box<dyn EventFunc>>,
+    accept_focus: Option<Box<dyn EventFunc<bool>>>,
+    adjust_child: Option<Box<dyn AdjustChildConstraintsFunc>>,
+    position_child: Option<Box<dyn PositionChildFunc>>,
+    hit_test: Option<Box<dyn OneParamEventFunc<Point<Px>, bool>>>,
+    hover: Option<Box<dyn OneParamEventFunc<Point<Px>>>>,
+    mouse_down:
+        Option<Box<dyn ThreeParamEventFunc<Point<Px>, DeviceId, MouseButton, EventHandling>>>,
+    mouse_drag: Option<Box<dyn ThreeParamEventFunc<Point<Px>, DeviceId, MouseButton>>>,
+    mouse_up: Option<Box<MouseUpFunc>>,
+    ime: Option<Box<dyn OneParamEventFunc<Ime, EventHandling>>>,
+    keyboard_input: Option<Box<dyn ThreeParamEventFunc<DeviceId, KeyEvent, bool, EventHandling>>>,
+    mouse_wheel:
+        Option<Box<dyn ThreeParamEventFunc<DeviceId, MouseScrollDelta, TouchPhase, EventHandling>>>,
+}
+
+impl Debug for Custom {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Custom")
+            .field("child", &self.child)
+            .finish_non_exhaustive()
+    }
+}
+
+impl Default for Custom {
+    fn default() -> Self {
+        Self::empty()
+    }
+}
+
+impl Custom {
+    /// Returns a custom widget that has no child.
+    pub fn empty() -> Self {
+        Self::new(Space::clear())
+    }
+
+    /// Returns a custom widget that contains `child`.
+    pub fn new(child: impl MakeWidget) -> Self {
+        Self {
+            child: WidgetRef::new(child),
+            redraw_background: None,
+            redraw_foreground: None,
+            background: None,
+            mounted: None,
+            unmounted: None,
+            unhover: None,
+            focus: None,
+            blur: None,
+            activate: None,
+            deactivate: None,
+            accept_focus: None,
+            adjust_child: None,
+            position_child: None,
+            hit_test: None,
+            hover: None,
+            mouse_down: None,
+            mouse_drag: None,
+            mouse_up: None,
+            ime: None,
+            keyboard_input: None,
+            mouse_wheel: None,
+        }
+    }
+
+    /// Sets the background color of this widget to `color` and returns self.
+    ///
+    /// If the color is set to a non-transparent value, it will be filled before
+    /// any of the redraw callbacks are invoked.
+    ///
+    /// This value coresponds to [`WrapperWidget::background_color`].
+    pub fn background_color(mut self, color: impl IntoValue<Color>) -> Self {
+        self.background = Some(color.into_value());
+        self
+    }
+
+    /// Sets `redraw` as the callback to invoke when redrawing this control.
+    ///
+    /// If this control contains a child, its redraw function will be invoked
+    /// after `redraw` is invoked. Use [`Self::on_redraw_after_child()`] to draw
+    /// after the child widget.
+    ///
+    /// This callback corresponds to [`WrapperWidget::redraw_background`].
+    pub fn on_redraw<Redraw>(mut self, redraw: Redraw) -> Self
+    where
+        Redraw: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window, 'clip, 'gfx, 'pass> FnMut(
+                &mut GraphicsContext<'context, 'window, 'clip, 'gfx, 'pass>,
+            ),
+    {
+        self.redraw_background = Some(Box::new(redraw));
+        self
+    }
+
+    /// Sets `redraw` as the callback to invoke when redrawing this control,
+    /// after the child control has been redrawn.
+    ///
+    /// If this control contains a child, its redraw function will be invoked
+    /// before `redraw` is invoked. Use [`Self::on_redraw()`] to draw before the
+    /// child widget.
+    ///
+    /// `redraw` will be invoked regardless of whether a child is present.
+    ///
+    /// This callback corresponds to [`WrapperWidget::redraw_foreground`].
+    pub fn on_redraw_after_child<Redraw>(mut self, redraw: Redraw) -> Self
+    where
+        Redraw: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window, 'clip, 'gfx, 'pass> FnMut(
+                &mut GraphicsContext<'context, 'window, 'clip, 'gfx, 'pass>,
+            ),
+    {
+        self.redraw_foreground = Some(Box::new(redraw));
+        self
+    }
+
+    /// Sets `mounted` to be invoked when this widget is mounted into a parent.
+    ///
+    /// This callback corresponds to [`WrapperWidget::mounted`].
+    pub fn on_mounted<Mounted>(mut self, mounted: Mounted) -> Self
+    where
+        Mounted: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>),
+    {
+        self.mounted = Some(Box::new(mounted));
+        self
+    }
+
+    /// Sets `unmounted` to be invoked when this widget is unmounted from its
+    /// parent.
+    ///
+    /// This callback corresponds to [`WrapperWidget::unmounted`].
+    pub fn on_unmounted<Mounted>(mut self, mounted: Mounted) -> Self
+    where
+        Mounted: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>),
+    {
+        self.unmounted = Some(Box::new(mounted));
+        self
+    }
+
+    /// Invokes `unhovered` when the mouse cursor leaves the widget's boundary.
+    ///
+    /// This callback corresponds to [`WrapperWidget::unhover`].
+    pub fn on_unhover<Unhover>(mut self, unhovered: Unhover) -> Self
+    where
+        Unhover: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>),
+    {
+        self.unhover = Some(Box::new(unhovered));
+        self
+    }
+
+    /// Invokes `focus` when the widget receives input focus.
+    ///
+    /// This callback corresponds to [`WrapperWidget::focus`].
+    pub fn on_focus<Focused>(mut self, focus: Focused) -> Self
+    where
+        Focused: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>),
+    {
+        self.focus = Some(Box::new(focus));
+        self
+    }
+
+    /// Invokes `blur` when the widget loses input focus.
+    ///
+    /// This callback corresponds to [`WrapperWidget::blur`].
+    pub fn on_blur<Blur>(mut self, blur: Blur) -> Self
+    where
+        Blur: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>),
+    {
+        self.blur = Some(Box::new(blur));
+        self
+    }
+
+    /// Invokes `activated` when this widget becomes the active widget.
+    ///
+    /// This callback corresponds to [`WrapperWidget::activate`].
+    pub fn on_activate<Activated>(mut self, activated: Activated) -> Self
+    where
+        Activated: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>),
+    {
+        self.activate = Some(Box::new(activated));
+        self
+    }
+
+    /// Invokes `deactivated` when this widget no longer is the active widget.
+    ///
+    /// This callback corresponds to [`WrapperWidget::deactivate`].
+    pub fn on_deactivate<Deactivated>(mut self, deactivated: Deactivated) -> Self
+    where
+        Deactivated: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>),
+    {
+        self.deactivate = Some(Box::new(deactivated));
+        self
+    }
+
+    /// Invokes `accept` when this widget is set to receive input focus. If this
+    /// function returns true, this widget will become the focused widget.
+    ///
+    /// This callback corresponds to [`WrapperWidget::accept_focus`].
+    pub fn on_accept_focus<AcceptFocus>(mut self, accept: AcceptFocus) -> Self
+    where
+        AcceptFocus: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>) -> bool,
+    {
+        self.accept_focus = Some(Box::new(accept));
+        self
+    }
+
+    /// Invokes `adjust_child_constraints` before measuring the child widget.
+    /// The returned constraints will be passed along to the child in its layout
+    /// function.
+    ///
+    /// This callback corresponds to [`WrapperWidget::adjust_child_constraints`].
+    pub fn on_adjust_child_constraints<AdjustChildConstraints>(
+        mut self,
+        adjust_child_constraints: AdjustChildConstraints,
+    ) -> Self
+    where
+        AdjustChildConstraints: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window, 'clip, 'gfx, 'pass> FnMut(
+                Size<ConstraintLimit>,
+                &mut LayoutContext<'context, 'window, 'clip, 'gfx, 'pass>,
+            ) -> Size<ConstraintLimit>,
+    {
+        self.adjust_child = Some(Box::new(adjust_child_constraints));
+        self
+    }
+
+    /// Invokes `position_child` to determine the position of a measured child.
+    ///
+    /// This callback corresponds to [`WrapperWidget::position_child`].
+    pub fn on_position_child<PositionChild>(mut self, position_child: PositionChild) -> Self
+    where
+        PositionChild: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window, 'clip, 'gfx, 'pass> FnMut(
+                Size<Px>,
+                Size<ConstraintLimit>,
+                &mut LayoutContext<'context, 'window, 'clip, 'gfx, 'pass>,
+            ) -> WrappedLayout,
+    {
+        self.position_child = Some(Box::new(position_child));
+        self
+    }
+
+    /// Invokes `hit_test` when determining if a location should be considered
+    /// interacting with this widget.
+    ///
+    /// This callback corresponds to [`WrapperWidget::hit_test`].
+    pub fn on_hit_test<HitTest>(mut self, hit_test: HitTest) -> Self
+    where
+        HitTest: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(Point<Px>, &mut EventContext<'context, 'window>) -> bool,
+    {
+        self.hit_test = Some(Box::new(hit_test));
+        self
+    }
+
+    /// Invokes `hover` when a mouse cursor is above this widget.
+    ///
+    /// This callback corresponds to [`WrapperWidget::hover`].
+    pub fn on_hover<Hover>(mut self, hover: Hover) -> Self
+    where
+        Hover: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(Point<Px>, &mut EventContext<'context, 'window>),
+    {
+        self.hover = Some(Box::new(hover));
+        self
+    }
+
+    /// Invokes `mouse_down` when a mouse button is pushed on a location where
+    /// [`Self::on_hit_test`] returned true.
+    ///
+    /// Returning [`HANDLED`](crate::widget::HANDLED) will set this widget as
+    /// the handler for the [`DeviceId`] and [`MouseButton`]. Future mouse
+    /// events for the same device and button will be sent to this widget's
+    /// [`Self::on_mouse_drag`] and [`Self::on_mouse_up`] callbacks.
+    ///
+    /// This callback corresponds to [`WrapperWidget::mouse_down`].
+    pub fn on_mouse_down<MouseDown>(mut self, mouse_down: MouseDown) -> Self
+    where
+        MouseDown: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(
+                Point<Px>,
+                DeviceId,
+                MouseButton,
+                &mut EventContext<'context, 'window>,
+            ) -> EventHandling,
+    {
+        self.mouse_down = Some(Box::new(mouse_down));
+        self
+    }
+
+    /// Invokes `mouse_drag` when the mouse cursor moves while a tracked button
+    /// is presed.
+    ///
+    /// This callback corresponds to [`WrapperWidget::mouse_drag`].
+    pub fn on_mouse_drag<MouseDrag>(mut self, mouse_drag: MouseDrag) -> Self
+    where
+        MouseDrag: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(
+                Point<Px>,
+                DeviceId,
+                MouseButton,
+                &mut EventContext<'context, 'window>,
+            ),
+    {
+        self.mouse_drag = Some(Box::new(mouse_drag));
+        self
+    }
+
+    /// Invokes `mouse_up` when a tracked mouse button is released.
+    ///
+    /// This callback corresponds to [`WrapperWidget::mouse_up`].
+    pub fn on_mouse_up<MouseUp>(mut self, mouse_up: MouseUp) -> Self
+    where
+        MouseUp: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(
+                Option<Point<Px>>,
+                DeviceId,
+                MouseButton,
+                &mut EventContext<'context, 'window>,
+            ),
+    {
+        self.mouse_up = Some(Box::new(mouse_up));
+        self
+    }
+
+    /// Invokes `ime` when an input manager event occurs.
+    ///
+    /// This callback corresponds to [`WrapperWidget::ime`].
+    pub fn on_ime<OnIme>(mut self, ime: OnIme) -> Self
+    where
+        OnIme: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(Ime, &mut EventContext<'context, 'window>) -> EventHandling,
+    {
+        self.ime = Some(Box::new(ime));
+        self
+    }
+
+    /// Invokes `keyboard_input` when a keyboard event occurs.
+    ///
+    /// This callback corresponds to [`WrapperWidget::keyboard_input`].
+    pub fn on_keyboard_input<KeyboardInput>(mut self, keyboard_input: KeyboardInput) -> Self
+    where
+        KeyboardInput: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(
+                DeviceId,
+                KeyEvent,
+                bool,
+                &mut EventContext<'context, 'window>,
+            ) -> EventHandling,
+    {
+        self.keyboard_input = Some(Box::new(keyboard_input));
+        self
+    }
+
+    /// Invokes `mouse_wheel` when a mouse wheel event occurs.
+    ///
+    /// This callback corresponds to [`WrapperWidget::mouse_wheel`].
+    pub fn mouse_wheel<MouseWheel>(mut self, mouse_wheel: MouseWheel) -> Self
+    where
+        MouseWheel: Send
+            + UnwindSafe
+            + 'static
+            + for<'context, 'window> FnMut(
+                DeviceId,
+                MouseScrollDelta,
+                TouchPhase,
+                &mut EventContext<'context, 'window>,
+            ) -> EventHandling,
+    {
+        self.mouse_wheel = Some(Box::new(mouse_wheel));
+        self
+    }
+}
+
+impl WrapperWidget for Custom {
+    fn child_mut(&mut self) -> &mut WidgetRef {
+        &mut self.child
+    }
+
+    fn redraw_background(&mut self, context: &mut GraphicsContext<'_, '_, '_, '_, '_>) {
+        if let Some(redraw) = &mut self.redraw_background {
+            redraw.invoke(context);
+        }
+    }
+
+    fn redraw_foreground(&mut self, context: &mut GraphicsContext<'_, '_, '_, '_, '_>) {
+        if let Some(redraw) = &mut self.redraw_foreground {
+            redraw.invoke(context);
+        }
+    }
+
+    fn adjust_child_constraints(
+        &mut self,
+        available_space: Size<ConstraintLimit>,
+        context: &mut LayoutContext<'_, '_, '_, '_, '_>,
+    ) -> Size<ConstraintLimit> {
+        if let Some(adjust_child) = &mut self.adjust_child {
+            adjust_child.invoke(available_space, context)
+        } else {
+            available_space
+        }
+    }
+
+    fn position_child(
+        &mut self,
+        size: Size<Px>,
+        available_space: Size<ConstraintLimit>,
+        context: &mut LayoutContext<'_, '_, '_, '_, '_>,
+    ) -> WrappedLayout {
+        if let Some(position_child) = &mut self.position_child {
+            position_child.invoke(size, available_space, context)
+        } else {
+            Size::new(
+                available_space
+                    .width
+                    .fit_measured(size.width, context.gfx.scale()),
+                available_space
+                    .height
+                    .fit_measured(size.height, context.gfx.scale()),
+            )
+            .into()
+        }
+    }
+
+    fn background_color(&mut self, context: &WidgetContext<'_, '_>) -> Option<Color> {
+        self.background.as_ref().map(|bg| bg.get_tracked(context))
+    }
+
+    fn mounted(&mut self, context: &mut EventContext<'_, '_>) {
+        if let Some(mounted) = &mut self.mounted {
+            mounted.invoke(context);
+        }
+    }
+
+    fn unmounted(&mut self, context: &mut EventContext<'_, '_>) {
+        if let Some(unmounted) = &mut self.unmounted {
+            unmounted.invoke(context);
+        }
+    }
+
+    fn hit_test(&mut self, location: Point<Px>, context: &mut EventContext<'_, '_>) -> bool {
+        if let Some(hit_test) = &mut self.hit_test {
+            hit_test.invoke(location, context)
+        } else {
+            false
+        }
+    }
+
+    fn hover(&mut self, location: Point<Px>, context: &mut EventContext<'_, '_>) {
+        if let Some(hover) = &mut self.hover {
+            hover.invoke(location, context);
+        }
+    }
+
+    fn unhover(&mut self, context: &mut EventContext<'_, '_>) {
+        if let Some(unhover) = &mut self.unhover {
+            unhover.invoke(context);
+        }
+    }
+
+    fn accept_focus(&mut self, context: &mut EventContext<'_, '_>) -> bool {
+        if let Some(accept_focus) = &mut self.accept_focus {
+            accept_focus.invoke(context)
+        } else {
+            false
+        }
+    }
+
+    fn focus(&mut self, context: &mut EventContext<'_, '_>) {
+        if let Some(focus) = &mut self.focus {
+            focus.invoke(context);
+        }
+    }
+
+    fn blur(&mut self, context: &mut EventContext<'_, '_>) {
+        if let Some(blur) = &mut self.blur {
+            blur.invoke(context);
+        }
+    }
+
+    fn activate(&mut self, context: &mut EventContext<'_, '_>) {
+        if let Some(activate) = &mut self.activate {
+            activate.invoke(context);
+        }
+    }
+
+    fn deactivate(&mut self, context: &mut EventContext<'_, '_>) {
+        if let Some(deactivate) = &mut self.deactivate {
+            deactivate.invoke(context);
+        }
+    }
+
+    fn mouse_down(
+        &mut self,
+        location: Point<Px>,
+        device_id: DeviceId,
+        button: MouseButton,
+        context: &mut EventContext<'_, '_>,
+    ) -> EventHandling {
+        if let Some(mouse_down) = &mut self.mouse_down {
+            mouse_down.invoke(location, device_id, button, context)
+        } else {
+            IGNORED
+        }
+    }
+
+    fn mouse_drag(
+        &mut self,
+        location: Point<Px>,
+        device_id: DeviceId,
+        button: MouseButton,
+        context: &mut EventContext<'_, '_>,
+    ) {
+        if let Some(mouse_drag) = &mut self.mouse_drag {
+            mouse_drag.invoke(location, device_id, button, context);
+        }
+    }
+
+    fn mouse_up(
+        &mut self,
+        location: Option<Point<Px>>,
+        device_id: DeviceId,
+        button: MouseButton,
+        context: &mut EventContext<'_, '_>,
+    ) {
+        if let Some(mouse_up) = &mut self.mouse_up {
+            mouse_up.invoke(location, device_id, button, context);
+        }
+    }
+
+    fn keyboard_input(
+        &mut self,
+        device_id: DeviceId,
+        input: KeyEvent,
+        is_synthetic: bool,
+        context: &mut EventContext<'_, '_>,
+    ) -> EventHandling {
+        if let Some(keyboard_input) = &mut self.keyboard_input {
+            keyboard_input.invoke(device_id, input, is_synthetic, context)
+        } else {
+            IGNORED
+        }
+    }
+
+    fn ime(&mut self, ime: Ime, context: &mut EventContext<'_, '_>) -> EventHandling {
+        if let Some(f) = &mut self.ime {
+            f.invoke(ime, context)
+        } else {
+            IGNORED
+        }
+    }
+
+    fn mouse_wheel(
+        &mut self,
+        device_id: DeviceId,
+        delta: MouseScrollDelta,
+        phase: TouchPhase,
+        context: &mut EventContext<'_, '_>,
+    ) -> EventHandling {
+        if let Some(mouse_wheel) = &mut self.mouse_wheel {
+            mouse_wheel.invoke(device_id, delta, phase, context)
+        } else {
+            IGNORED
+        }
+    }
+}
+
+trait RedrawFunc: Send + UnwindSafe {
+    fn invoke(&mut self, context: &mut GraphicsContext<'_, '_, '_, '_, '_>);
+}
+
+impl<Func> RedrawFunc for Func
+where
+    Func: Send
+        + UnwindSafe
+        + 'static
+        + for<'context, 'window, 'clip, 'gfx, 'pass> FnMut(
+            &mut GraphicsContext<'context, 'window, 'clip, 'gfx, 'pass>,
+        ),
+{
+    fn invoke(&mut self, context: &mut GraphicsContext<'_, '_, '_, '_, '_>) {
+        self(context);
+    }
+}
+
+trait AdjustChildConstraintsFunc: Send + UnwindSafe {
+    fn invoke(
+        &mut self,
+        available_space: Size<ConstraintLimit>,
+        context: &mut LayoutContext<'_, '_, '_, '_, '_>,
+    ) -> Size<ConstraintLimit>;
+}
+
+impl<Func> AdjustChildConstraintsFunc for Func
+where
+    Func: Send
+        + UnwindSafe
+        + 'static
+        + for<'context, 'window, 'clip, 'gfx, 'pass> FnMut(
+            Size<ConstraintLimit>,
+            &mut LayoutContext<'context, 'window, 'clip, 'gfx, 'pass>,
+        ) -> Size<ConstraintLimit>,
+{
+    fn invoke(
+        &mut self,
+        available_space: Size<ConstraintLimit>,
+        context: &mut LayoutContext<'_, '_, '_, '_, '_>,
+    ) -> Size<ConstraintLimit> {
+        self(available_space, context)
+    }
+}
+
+trait PositionChildFunc: Send + UnwindSafe {
+    fn invoke(
+        &mut self,
+        size: Size<Px>,
+        available_space: Size<ConstraintLimit>,
+        context: &mut LayoutContext<'_, '_, '_, '_, '_>,
+    ) -> WrappedLayout;
+}
+
+impl<Func> PositionChildFunc for Func
+where
+    Func: Send
+        + UnwindSafe
+        + 'static
+        + for<'context, 'window, 'clip, 'gfx, 'pass> FnMut(
+            Size<Px>,
+            Size<ConstraintLimit>,
+            &mut LayoutContext<'context, 'window, 'clip, 'gfx, 'pass>,
+        ) -> WrappedLayout,
+{
+    fn invoke(
+        &mut self,
+        size: Size<Px>,
+        available_space: Size<ConstraintLimit>,
+        context: &mut LayoutContext<'_, '_, '_, '_, '_>,
+    ) -> WrappedLayout {
+        self(size, available_space, context)
+    }
+}
+
+trait EventFunc<R = ()>: Send + UnwindSafe {
+    fn invoke(&mut self, context: &mut EventContext<'_, '_>) -> R;
+}
+
+impl<R, Func> EventFunc<R> for Func
+where
+    Func: Send
+        + UnwindSafe
+        + 'static
+        + for<'context, 'window> FnMut(&mut EventContext<'context, 'window>) -> R,
+{
+    fn invoke(&mut self, context: &mut EventContext<'_, '_>) -> R {
+        self(context)
+    }
+}
+
+trait OneParamEventFunc<P, R = ()>: Send + UnwindSafe {
+    fn invoke(&mut self, param: P, context: &mut EventContext<'_, '_>) -> R;
+}
+
+impl<P, R, Func> OneParamEventFunc<P, R> for Func
+where
+    Func: Send
+        + UnwindSafe
+        + 'static
+        + for<'context, 'window> FnMut(P, &mut EventContext<'context, 'window>) -> R,
+{
+    fn invoke(&mut self, location: P, context: &mut EventContext<'_, '_>) -> R {
+        self(location, context)
+    }
+}
+
+trait ThreeParamEventFunc<P1, P2, P3, R = ()>: Send + UnwindSafe {
+    fn invoke(
+        &mut self,
+        location: P1,
+        device_id: P2,
+        button: P3,
+        context: &mut EventContext<'_, '_>,
+    ) -> R;
+}
+
+type MouseUpFunc = dyn ThreeParamEventFunc<Option<Point<Px>>, DeviceId, MouseButton>;
+
+impl<P1, P2, P3, R, Func> ThreeParamEventFunc<P1, P2, P3, R> for Func
+where
+    Func: Send
+        + UnwindSafe
+        + 'static
+        + for<'context, 'window> FnMut(P1, P2, P3, &mut EventContext<'context, 'window>) -> R,
+{
+    fn invoke(
+        &mut self,
+        location: P1,
+        device_id: P2,
+        button: P3,
+        context: &mut EventContext<'_, '_>,
+    ) -> R {
+        self(location, device_id, button, context)
+    }
+}
diff --git a/src/widgets/label.rs b/src/widgets/label.rs
index 2cf549c..7d040f4 100644
--- a/src/widgets/label.rs
+++ b/src/widgets/label.rs
@@ -40,7 +40,7 @@ impl Label {
                 if *prepared_generation == check_generation
                     && *prepared_color == color
                     && (*prepared_width == width
-                        || (*prepared_width < width
+                        || ((*prepared_width < width || prepared.size.width <= width)
                             && prepared.line_height == prepared.size.height)) => {}
             _ => {
                 let measured = self.text.map(|text| {
diff --git a/src/widgets/space.rs b/src/widgets/space.rs
index 0befe0c..a5931a0 100644
--- a/src/widgets/space.rs
+++ b/src/widgets/space.rs
@@ -41,7 +41,7 @@ impl Widget for Space {
     fn redraw(&mut self, context: &mut GraphicsContext<'_, '_, '_, '_, '_>) {
         self.color.redraw_when_changed(context);
         let color = self.color.get();
-        context.gfx.fill(color);
+        context.fill(color);
     }
 
     fn layout(
diff --git a/src/widgets/stack.rs b/src/widgets/stack.rs
index 097b368..7f5a805 100644
--- a/src/widgets/stack.rs
+++ b/src/widgets/stack.rs
@@ -5,7 +5,9 @@ use std::ops::{Bound, Deref};
 
 use alot::{LotId, OrderedLots};
 use kludgine::figures::units::{Lp, UPx};
-use kludgine::figures::{Fraction, IntoSigned, IntoUnsigned, Point, Rect, ScreenScale, Size};
+use kludgine::figures::{
+    Fraction, IntoSigned, IntoUnsigned, Point, Rect, Round, ScreenScale, Size,
+};
 
 use crate::context::{AsEventContext, EventContext, GraphicsContext, LayoutContext};
 use crate::styles::Dimension;
@@ -343,7 +345,7 @@ impl Layout {
                 self.premeasured.retain(|&measured| measured != id);
                 match min {
                     Dimension::Px(pixels) => {
-                        self.allocated_space.0 -= pixels.into_unsigned();
+                        self.allocated_space.0 -= pixels.into_unsigned().ceil();
                     }
                     Dimension::Lp(lp) => {
                         self.allocated_space.1 -= lp;
@@ -403,7 +405,8 @@ impl Layout {
     ) -> Size<UPx> {
         let (space_constraint, other_constraint) = self.orientation.split_size(available);
         let available_space = space_constraint.max();
-        let allocated_space = self.allocated_space.0 + self.allocated_space.1.into_upx(scale);
+        let allocated_space =
+            self.allocated_space.0 + self.allocated_space.1.into_upx(scale).ceil();
         let mut remaining = available_space.saturating_sub(allocated_space);
         // If our `other_constraint` is not known, we will need to give child
         // widgets an opportunity to lay themselves out in the full area. This
@@ -442,8 +445,8 @@ impl Layout {
 
         // Measure the weighted children within the remaining space
         if self.total_weights > 0 {
-            let space_per_weight = remaining / self.total_weights;
-            remaining %= self.total_weights;
+            let space_per_weight = (remaining / self.total_weights).floor();
+            remaining -= space_per_weight * self.total_weights;
             for (fractional_index, &(id, weight)) in self.fractional.iter().enumerate() {
                 let index = self.children.index_of_id(id).expect("child not found");
                 let mut size = space_per_weight * u32::from(weight);
@@ -453,7 +456,7 @@ impl Layout {
                     let from_end = u32::try_from(self.fractional.len() - fractional_index)
                         .expect("too many items");
                     if remaining >= from_end {
-                        let amount = (remaining + from_end - 1) / from_end;
+                        let amount = (remaining / from_end).ceil().min(remaining);
                         remaining -= amount;
                         size += amount;
                     }
diff --git a/src/widgets/switcher.rs b/src/widgets/switcher.rs
index d4b9ee9..a1b5371 100644
--- a/src/widgets/switcher.rs
+++ b/src/widgets/switcher.rs
@@ -49,7 +49,7 @@ impl WrapperWidget for Switcher {
     }
 
     // TODO this should be moved to an invalidated() event once we have it.
-    fn adjust_child_constraint(
+    fn adjust_child_constraints(
         &mut self,
         available_space: Size<ConstraintLimit>,
         context: &mut LayoutContext<'_, '_, '_, '_, '_>,