Announce pressed state of selectable buttons to screen readers (#8130)

A `Button` used as a toggle (via `Button::selected(true)`,
`Button::selectable(...)`, or `Ui::selectable_label`) now announces its
pressed / not-pressed state to screen readers. Plain buttons that never
call `.selected(...)` stay un-toggled, so their announcement doesn't
change. `Checkbox` and the pre-existing selectable-label code path
already did this; `Button` was the odd one out.

No public API change: the field is private, `Button::selected(bool)`
keeps its signature, and visuals are identical. Internally the field
becomes `Option<bool>` so we can distinguish "plain button" from "toggle
button currently off".

Regression test added in `regression_tests.rs`, do let me know if some
other file would be a better location.

### Note for manual testing

`egui_demo_app` pulls in `eframe` with `default-features = false` and
doesn't re-enable `accesskit`, so `cargo run -p egui_demo_app` publishes
no AccessKit tree at all. To verify manually:
`cargo run -p egui_demo_app --features accessibility_inspector`
Happy to send a small follow-up PR enabling `accesskit` in the demo
app's defaults if that's desirable, since that makes a11y work much
easier to smoke-test locally.

### Use of AI

This PR was drafted with Claude Code. I understand different projects
have different policies regarding AI generated code. Do let me know if
this is not acceptable here. Also happy to take any other feedback.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/egui/src/widgets/button.rs

@@ -35,7 +35,7 @@ pub struct Button<'a> {
     frame_when_inactive: bool,
     min_size: Vec2,
     corner_radius: Option<CornerRadius>,
-    selected: bool,
+    selected: Option<bool>,
     image_tint_follows_text_color: bool,
     limit_image_size: bool,
     classes: Classes,
@@ -54,7 +54,7 @@ impl<'a> Button<'a> {
             frame_when_inactive: true,
             min_size: Vec2::ZERO,
             corner_radius: None,
-            selected: false,
+            selected: None,
             image_tint_follows_text_color: false,
             limit_image_size: false,
             classes: Classes::default(),
@@ -261,9 +261,14 @@ impl<'a> Button<'a> {
     }
 
     /// If `true`, mark this button as "selected".
+    ///
+    /// Calling this method opts the button into toggle semantics and the
+    /// current pressed/not-pressed state will be reported to assistive
+    /// technologies (e.g. screen readers). Plain buttons that never call
+    /// `selected` are not announced as toggles.
     #[inline]
     pub fn selected(mut self, selected: bool) -> Self {
-        self.selected = selected;
+        self.selected = Some(selected);
         self
     }
 
@@ -314,7 +319,7 @@ 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_CLASS, selected);
+        classes.add_class_if(SELECTED_CLASS, selected.unwrap_or(false));
 
         let ButtonStyle { frame, text_style } = ui.style().button_style(&classes, state);
 
@@ -376,12 +381,18 @@ impl<'a> Button<'a> {
             ui.ctx().set_cursor_icon(cursor);
         }
 
-        response.response.widget_info(|| {
-            if let Some(text) = &text {
-                WidgetInfo::labeled(WidgetType::Button, ui.is_enabled(), text)
-            } else {
-                WidgetInfo::new(WidgetType::Button)
+        response.response.widget_info(|| match (selected, &text) {
+            (Some(selected), Some(text)) => {
+                WidgetInfo::selected(WidgetType::Button, ui.is_enabled(), selected, text)
             }
+            (Some(selected), None) => {
+                let mut info = WidgetInfo::new(WidgetType::Button);
+                info.enabled = ui.is_enabled();
+                info.selected = Some(selected);
+                info
+            }
+            (None, Some(text)) => WidgetInfo::labeled(WidgetType::Button, ui.is_enabled(), text),
+            (None, None) => WidgetInfo::new(WidgetType::Button),
         });
 
         response

tests/egui_tests/tests/regression_tests.rs

@@ -6,11 +6,11 @@ use egui::epaint::Shape;
 use egui::style::ScrollAnimation;
 use egui::text::{LayoutJob, TextWrapping};
 use egui::{
-    Align, Color32, FontFamily, FontId, Image, Label, Layout, RichText, Sense, TextBuffer,
+    Align, Button, Color32, FontFamily, FontId, Image, Label, Layout, RichText, Sense, TextBuffer,
     TextFormat, TextWrapMode, Ui, include_image, vec2,
 };
 use egui_kittest::Harness;
-use egui_kittest::kittest::Queryable as _;
+use egui_kittest::kittest::{NodeT as _, Queryable as _};
 
 #[test]
 fn image_button_should_have_alt_text() {
@@ -23,6 +23,31 @@ fn image_button_should_have_alt_text() {
     harness.get_by_label("Egui");
 }
 
+#[test]
+fn button_selected_should_announce_toggled_state() {
+    use egui::accesskit::Toggled;
+
+    let harness = Harness::new_ui(|ui| {
+        ui.add(Button::new("Plain"));
+        ui.add(Button::new("Off").selected(false));
+        ui.add(Button::new("On").selected(true));
+    });
+
+    assert_eq!(
+        harness.get_by_label("Plain").accesskit_node().toggled(),
+        None,
+        "a plain Button must not be announced as a toggle",
+    );
+    assert_eq!(
+        harness.get_by_label("Off").accesskit_node().toggled(),
+        Some(Toggled::False),
+    );
+    assert_eq!(
+        harness.get_by_label("On").accesskit_node().toggled(),
+        Some(Toggled::True),
+    );
+}
+
 #[test]
 fn hovering_should_preserve_text_format() {
     let mut harness = Harness::builder().with_size((200.0, 70.0)).build_ui(|ui| {