Let an Area carry classes, and mark menu popups with MENU_CLASS

crates/egui/src/containers/area.rs

@@ -7,6 +7,7 @@ use emath::GuiRounding as _;
 use crate::{
     Align2, Context, Id, InnerResponse, LayerId, Layout, NumExt as _, Order, Pos2, Rect, Response,
     Sense, Ui, UiBuilder, UiKind, UiStackInfo, Vec2, WidgetRect, WidgetWithState, emath, pos2,
+    widget_style::{Classes, HasClasses},
 };
 
 /// State of an [`Area`] that is persisted between frames.
@@ -122,12 +123,23 @@ pub struct Area {
     fade_in: bool,
     layout: Layout,
     sizing_pass: bool,
+    classes: Classes,
 }
 
 impl WidgetWithState for Area {
     type State = AreaState;
 }
 
+impl HasClasses for Area {
+    fn classes(&self) -> &Classes {
+        &self.classes
+    }
+
+    fn classes_mut(&mut self) -> &mut Classes {
+        &mut self.classes
+    }
+}
+
 impl Area {
     /// The `id` must be globally unique.
     pub fn new(id: Id) -> Self {
@@ -149,6 +161,7 @@ impl Area {
             fade_in: true,
             layout: Layout::default(),
             sizing_pass: false,
+            classes: Classes::default(),
         }
     }
 
@@ -400,6 +413,7 @@ pub(crate) struct Prepared {
 
     fade_in: bool,
     layout: Layout,
+    classes: Classes,
 }
 
 impl Area {
@@ -434,6 +448,7 @@ impl Area {
             fade_in,
             layout,
             sizing_pass: force_sizing_pass,
+            classes,
         } = self;
 
         let constrain_rect = constrain_rect.unwrap_or_else(|| ctx.content_rect());
@@ -581,6 +596,7 @@ impl Area {
             sizing_pass,
             fade_in,
             layout,
+            classes,
         }
     }
 }
@@ -612,6 +628,7 @@ impl Prepared {
         let max_rect = self.state.rect();
 
         let mut ui_builder = UiBuilder::new()
+            .with_classes(core::mem::take(&mut self.classes))
             .ui_stack_info(self.info.take().unwrap_or_default())
             .layer_id(self.layer_id)
             .max_rect(max_rect)

crates/egui/src/containers/popup.rs

@@ -7,6 +7,7 @@ use crate::{
     Sense, Ui, UiKind, UiStackInfo,
     containers::menu::{MenuConfig, MenuState, menu_style},
     style::StyleModifier,
+    widget_style::{HasClasses as _, MENU_CLASS},
 };
 
 /// What should we anchor the popup to?
@@ -594,6 +595,7 @@ impl<'a> Popup<'a> {
         let (pivot, anchor) = best_align.pivot_pos(&anchor_rect, gap);
 
         let mut area = Area::new(id)
+            .with_class_if(MENU_CLASS, kind == PopupKind::Menu)
             .order(kind.order())
             .pivot(pivot)
             .fixed_pos(anchor)

crates/egui/src/widget_style/classes.rs

@@ -13,6 +13,12 @@ pub const SELECTED_CLASS: &str = "selected";
 /// The read-only class is present on a [`crate::TextEdit`] whose buffer can't be edited.
 pub const READ_ONLY_CLASS: &str = "read-only";
 
+/// The menu class is present on the [`crate::Ui`] of a menu popup, and so on every widget in it.
+///
+/// A menu lays its items out itself, so a widget in one is generally styled to fit that layout
+/// rather than to stand on its own.
+pub const MENU_CLASS: &str = "menu";
+
 /// A class is a static string identifier.
 pub type ClassName = Cow<'static, str>;
 

crates/egui/src/widget_style/mod.rs

@@ -5,7 +5,7 @@
 mod classes;
 
 pub use self::classes::{
-    ClassName, Classes, HasClasses, READ_ONLY_CLASS, ROOT_CLASS, SELECTED_CLASS,
+    ClassName, Classes, HasClasses, MENU_CLASS, READ_ONLY_CLASS, ROOT_CLASS, SELECTED_CLASS,
 };
 
 use core::fmt::Debug;