Add ability to highlight any widget (#2632)

* Add ability to highlight any widget

* Add line to changelog

* Demote the demo to a test

crates/egui_demo_lib/src/demo/demo_app_windows.rs

@@ -90,6 +90,7 @@ impl Default for Tests {
     fn default() -> Self {
         Self::from_demos(vec![
             Box::new(super::tests::CursorTest::default()),
+            Box::new(super::highlighting::Highlighting::default()),
             Box::new(super::tests::IdTest::default()),
             Box::new(super::tests::InputTest::default()),
             Box::new(super::layout_test::LayoutTest::default()),

crates/egui_demo_lib/src/demo/highlighting.rs

@@ -0,0 +1,37 @@
+#[derive(Default)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "serde", serde(default))]
+pub struct Highlighting {}
+
+impl super::Demo for Highlighting {
+    fn name(&self) -> &'static str {
+        "Highlighting"
+    }
+
+    fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
+        egui::Window::new(self.name())
+            .default_width(320.0)
+            .open(open)
+            .show(ctx, |ui| {
+                use super::View as _;
+                self.ui(ui);
+            });
+    }
+}
+
+impl super::View for Highlighting {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        ui.label("This demo demonstrates highlighting a widget.");
+        ui.add_space(4.0);
+        let label_response = ui.label("Hover me to highlight the button!");
+        ui.add_space(4.0);
+        let mut button_response = ui.button("Hover the button to highlight the label!");
+
+        if label_response.hovered() {
+            button_response = button_response.highlight();
+        }
+        if button_response.hovered() {
+            label_response.highlight();
+        }
+    }
+}

crates/egui_demo_lib/src/demo/mod.rs

@@ -12,6 +12,7 @@ pub mod dancing_strings;
 pub mod demo_app_windows;
 pub mod drag_and_drop;
 pub mod font_book;
+pub mod highlighting;
 pub mod layout_test;
 pub mod misc_demo_window;
 pub mod multi_touch;