Add a PopupStyle, so the theme frames popups and menus

A `Popup` framed itself with `Frame::popup` and left item spacing to the
ambient style. Both now come from a `PopupStyle`, which is what lets a theme
give a menu its own padding and its own item rhythm.

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

crates/egui/src/containers/popup.rs

@@ -7,7 +7,7 @@ use crate::{
     Sense, Ui, UiKind, UiStackInfo,
     containers::menu::{MenuConfig, MenuState, menu_style},
     style::StyleModifier,
-    widget_style::HasClasses as _,
+    widget_style::{Classes, HasClasses as _, PopupStyle},
 };
 
 /// What should we anchor the popup to?
@@ -600,8 +600,10 @@ impl<'a> Popup<'a> {
 
         let (pivot, anchor) = best_align.pivot_pos(&anchor_rect, gap);
 
+        let classes = Classes::default().with_class_if(Self::CLASS_MENU, kind == PopupKind::Menu);
+
         let mut area = Area::new(id)
-            .with_class_if(Self::CLASS_MENU, kind == PopupKind::Menu)
+            .with_classes(classes.clone())
             .order(kind.order())
             .pivot(pivot)
             .fixed_pos(anchor)
@@ -624,7 +626,12 @@ impl<'a> Popup<'a> {
 
         let mut response = area.show(&ctx, |ui| {
             style.apply(ui.style_mut());
-            let frame = frame.unwrap_or_else(|| Frame::popup(ui.style()));
+
+            // The theme decides how the popup is framed and how tightly its items sit together.
+            let popup_style: PopupStyle = ui.widget_style(id, &classes);
+            ui.spacing_mut().item_spacing = popup_style.item_spacing;
+
+            let frame = frame.unwrap_or(popup_style.frame);
             frame.show(ui, content).inner
         });
 

crates/egui/src/theme/default_style.rs

@@ -5,8 +5,8 @@ use crate::{
     Button, Context, Frame, TextEdit, TextStyle,
     theme::StyleProvider,
     widget_style::{
-        AtomLayoutStyle, ButtonStyle, CheckboxStyle, HasClasses as _, SeparatorStyle, StyleArgs,
-        TextEditStyle, TextVisuals, WidgetState,
+        AtomLayoutStyle, ButtonStyle, CheckboxStyle, HasClasses as _, PopupStyle, SeparatorStyle,
+        StyleArgs, TextEditStyle, TextVisuals, WidgetState,
     },
 };
 
@@ -26,6 +26,7 @@ impl DefaultStyle {
             ctx.themes.register::<SeparatorStyle>(Self, false);
             ctx.themes.register::<CheckboxStyle>(Self, false);
             ctx.themes.register::<TextEditStyle>(Self, false);
+            ctx.themes.register::<PopupStyle>(Self, false);
         });
     }
 }
@@ -105,6 +106,17 @@ impl StyleProvider<ButtonStyle> for DefaultStyle {
     }
 }
 
+impl StyleProvider<PopupStyle> for DefaultStyle {
+    fn style(&mut self, modifiers: &StyleArgs<'_>) -> PopupStyle {
+        let StyleArgs { style, .. } = modifiers;
+
+        PopupStyle {
+            frame: Frame::popup(style),
+            item_spacing: style.spacing.item_spacing,
+        }
+    }
+}
+
 impl StyleProvider<TextEditStyle> for DefaultStyle {
     fn style(&mut self, modifiers: &StyleArgs<'_>) -> TextEditStyle {
         let StyleArgs {

crates/egui/src/widget_style/mod.rs

@@ -136,6 +136,21 @@ pub struct ButtonStyle {
 
 impl WidgetStyle for ButtonStyle {}
 
+/// Dedicated style for a [`crate::Popup`], including menus and tooltips
+#[derive(Debug, Clone)]
+pub struct PopupStyle {
+    /// Frame around the popup's contents, including its padding.
+    pub frame: Frame,
+
+    /// Spacing between the items inside the popup.
+    ///
+    /// A menu wants its items flush against each other, while a tooltip wants them spaced out
+    /// like any other content.
+    pub item_spacing: Vec2,
+}
+
+impl WidgetStyle for PopupStyle {}
+
 /// Dedicated text edit style
 #[derive(Debug, Clone)]
 pub struct TextEditStyle {