Let the theme set a button's atom gap

`Button` hard-coded `spacing.icon_spacing` as the gap between its icon and
its text. A theme that gives a button its own metrics wants to set that gap
too, so it becomes a `ButtonStyle` field, overridden by `Button::gap`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui/src/atomics/atom_layout.rs

@@ -107,6 +107,13 @@ impl<'a> AtomLayout<'a> {
         self
     }
 
+    /// Set the gap between atoms, unless [`Self::gap`] already set one.
+    #[inline]
+    pub fn fallback_gap(mut self, gap: f32) -> Self {
+        self.gap.get_or_insert(gap);
+        self
+    }
+
     /// Set the [`Frame`].
     #[inline]
     pub fn frame(mut self, frame: Frame) -> Self {

crates/egui/src/theme/default_style.rs

@@ -91,6 +91,7 @@ impl StyleProvider<ButtonStyle> for DefaultStyle {
             // Historically only the height was floored, so that a button is at least as tall as
             // any other interactive widget on the same row.
             min_size: Vec2::new(0.0, spacing.interact_size.y),
+            gap: spacing.icon_spacing,
         }
     }
 }

crates/egui/src/widget_style/mod.rs

@@ -58,6 +58,11 @@ pub struct ButtonStyle {
     /// Ignored by a [`crate::Button::small`] button, which sizes itself purely from its contents
     /// and its own [`crate::Button::min_size`].
     pub min_size: Vec2,
+
+    /// The gap between the button's atoms, e.g. between its icon and its text.
+    ///
+    /// Overridden by [`crate::Button::gap`].
+    pub gap: f32,
 }
 
 impl WidgetStyle for ButtonStyle {}

crates/egui/src/widgets/button.rs

@@ -327,8 +327,11 @@ impl<'a> Button<'a> {
             frame,
             text_style,
             min_size: style_min_size,
+            gap,
         } = ui.widget_style(id, &classes);
 
+        layout = layout.fallback_gap(gap);
+
         // The theme decides how small a button may get — unless it is a `small` one, which sizes
         // itself purely from its contents.
         if !small {

examples/styling_engine/src/main.rs

@@ -69,6 +69,7 @@ impl StyleProvider<ButtonStyle> for MyTheme {
                 .inner_margin(8),
             text_style: base.text,
             min_size: args.style.spacing.interact_size,
+            gap: args.style.spacing.icon_spacing,
         }
     }
 }