Make Button sizing strategy adaptive

core/src/length.rs

@@ -42,6 +42,18 @@ impl Length {
     pub fn is_fill(&self) -> bool {
         self.fill_factor() != 0
     }
+
+    /// Returns the "fluid" variant of the [`Length`].
+    ///
+    /// Specifically:
+    /// - [`Length::Shrink`] if [`Length::Shrink`] or [`Length::Fixed`].
+    /// - [`Length::Fill`] otherwise.
+    pub fn fluid(&self) -> Length {
+        match self {
+            Length::Fill | Length::FillPortion(_) => Length::Fill,
+            Length::Shrink | Length::Fixed(_) => Length::Shrink,
+        }
+    }
 }
 
 impl From<Pixels> for Length {

widget/src/button.rs

@@ -71,11 +71,14 @@ where
 {
     /// Creates a new [`Button`] with the given content.
     pub fn new(content: impl Into<Element<'a, Message, Renderer>>) -> Self {
+        let content = content.into();
+        let size = content.as_widget().size_hint();
+
         Button {
-            content: content.into(),
+            content,
             on_press: None,
-            width: Length::Shrink,
-            height: Length::Shrink,
+            width: size.width.fluid(),
+            height: size.height.fluid(),
             padding: Padding::new(5.0),
             style: <Renderer::Theme as StyleSheet>::Style::default(),
         }

widget/src/container.rs

@@ -52,16 +52,8 @@ where
         Container {
             id: None,
             padding: Padding::ZERO,
-            width: if size.width.is_fill() {
-                Length::Fill
-            } else {
-                Length::Shrink
-            },
-            height: if size.height.is_fill() {
-                Length::Fill
-            } else {
-                Length::Shrink
-            },
+            width: size.width.fluid(),
+            height: size.height.fluid(),
             max_width: f32::INFINITY,
             max_height: f32::INFINITY,
             horizontal_alignment: alignment::Horizontal::Left,