fix: Smallvec, const string select

crates/egui/src/ui.rs

@@ -140,7 +140,7 @@ impl Ui {
         let disabled = disabled || invisible;
         let style = style.unwrap_or_else(|| ctx.global_style());
         let sense = sense.unwrap_or_else(Sense::hover);
-        let classes = classes.unwrap_or_default().with_class(ROOT_CLASS);
+        let classes = classes.with_class(ROOT_CLASS);
 
         let placer = Placer::new(max_rect, layout);
         let ui_stack = UiStack {
@@ -284,7 +284,6 @@ impl Ui {
         let sizing_pass = self.sizing_pass || sizing_pass;
         let style = style.unwrap_or_else(|| Arc::clone(&self.style));
         let sense = sense.unwrap_or_else(Sense::hover);
-        let classes = classes.unwrap_or_default();
 
         if sizing_pass {
             // During the sizing pass we want widgets to use up as little space as possible,

crates/egui/src/ui_builder.rs

@@ -2,6 +2,7 @@ use std::{hash::Hash, sync::Arc};
 
 #[expect(unused_imports)] // Used for doclinks
 use crate::Ui;
+use crate::widget_style::HasClasses;
 use crate::{ClosableTag, widget_style::Classes};
 use crate::{Id, LayerId, Layout, Rect, Sense, Style, UiStackInfo};
 
@@ -25,7 +26,7 @@ pub struct UiBuilder {
     pub style: Option<Arc<Style>>,
     pub sense: Option<Sense>,
     pub accessibility_parent: Option<Id>,
-    pub classes: Option<Classes>,
+    pub classes: Classes,
 }
 
 impl UiBuilder {
@@ -192,11 +193,14 @@ impl UiBuilder {
         self.accessibility_parent = Some(parent_id);
         self
     }
+}
 
-    /// Set classes for this [`Ui`].
-    #[inline]
-    pub fn classes(mut self, classes: Classes) -> Self {
-        self.classes = Some(classes);
-        self
+impl HasClasses for UiBuilder {
+    fn classes(&self) -> &Classes {
+        &self.classes
+    }
+
+    fn classes_mut(&mut self) -> &mut Classes {
+        &mut self.classes
     }
 }

crates/egui/src/ui_stack.rs

@@ -202,7 +202,7 @@ impl UiTags {
 /// Note: since [`UiStack`] contains a reference to its parent, it is both a stack, and a node within
 /// that stack. Most of its methods are about the specific node, but some methods walk up the
 /// hierarchy to provide information about the entire stack.
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub struct UiStack {
     // stuff that `Ui::child_ui` can deal with directly
     pub id: Id,

crates/egui/src/widget_style.rs

@@ -2,6 +2,7 @@ use std::{borrow::Cow, fmt};
 
 use emath::Vec2;
 use epaint::{Color32, FontId, Shadow, Stroke, text::TextWrapMode};
+use smallvec::SmallVec;
 
 use crate::{
     Frame, Response, Style, TextBuffer as _, TextStyle,
@@ -219,13 +220,14 @@ impl Style {
 
 /// The root class is present on every top-level ui
 pub const ROOT_CLASS: &str = "root";
+pub const SELECTED_CLASS: &str = "selected";
 
 pub type ClassName = Cow<'static, str>;
 
 /// The classes assigned to a widget
 #[derive(Debug, Default, Clone)]
 pub struct Classes {
-    classes: Vec<ClassName>,
+    classes: SmallVec<[ClassName; 5]>,
 }
 
 impl Classes {

crates/egui/src/widgets/button.rs

@@ -4,7 +4,7 @@ use crate::{
     Atom, AtomExt as _, AtomKind, AtomLayout, AtomLayoutResponse, Color32, CornerRadius, Frame,
     Image, IntoAtoms, NumExt as _, Response, Sense, Stroke, TextStyle, TextWrapMode, Ui, Vec2,
     Widget, WidgetInfo, WidgetText, WidgetType,
-    widget_style::{ButtonStyle, Classes, HasClasses, WidgetState},
+    widget_style::{ButtonStyle, Classes, HasClasses, SELECTED_CLASS, WidgetState},
 };
 
 /// Clickable button with text.
@@ -308,7 +308,7 @@ impl<'a> Button<'a> {
         let response: Option<Response> = ui.ctx().read_response(id);
         let state = response.map(|r| r.widget_state()).unwrap_or_default();
 
-        classes.add_class_if("selected", selected);
+        classes.add_class_if(SELECTED_CLASS, selected);
 
         let ButtonStyle { frame, text_style } = ui.style().button_style(&classes, state);