Add UiStack::has_class and StyleArgs::has_class

A `StyleProvider` reads a class either off the widget or off the `Ui`s it
sits in — the equivalent of a descendant selector. Give it one call that
covers both, next to the classes it queries.

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

crates/egui/src/ui_stack.rs

@@ -1,7 +1,7 @@
 use core::{any::Any, iter::FusedIterator};
 use std::sync::Arc;
 
-use crate::widget_style::Classes;
+use crate::widget_style::{ClassName, Classes, HasClasses as _};
 use epaint::Color32;
 
 use crate::{Direction, Frame, Id, Rect};
@@ -290,6 +290,13 @@ impl UiStack {
     pub fn contained_in(&self, kind: UiKind) -> bool {
         self.iter().any(|frame| frame.kind() == Some(kind))
     }
+
+    /// Does this node, or any [`crate::Ui`] up the stack, carry this class?
+    pub fn has_class(&self, class: impl Into<ClassName>) -> bool {
+        let class = class.into();
+        self.iter()
+            .any(|node| node.classes.has_class(class.clone()))
+    }
 }
 
 // ----------------------------------------------------------------------------

crates/egui/src/widget_style/mod.rs

@@ -149,3 +149,15 @@ pub struct StyleArgs<'a> {
     pub style: &'a Style,
     pub ctx: &'a Context,
 }
+
+impl StyleArgs<'_> {
+    /// Does the widget or any of its parents contain this class?
+    ///
+    /// See also:
+    /// - [`Classes::has_class`]
+    /// - [`UiStack::has_class`]
+    pub fn has_class(&self, class: impl Into<ClassName>) -> bool {
+        let class = class.into();
+        self.classes.has_class(class.clone()) || self.stack.has_class(class)
+    }
+}