`ComboBox`: fix justified layout of popup if wider than parent button (#4570)

* Closes https://github.com/emilk/egui/issues/4452

The `ComboBox` popup has a justified layout to make selection of items
easier.

Thanks to [the new sizing pass
logic](https://github.com/emilk/egui/issues/4535) we don't have to know
the final width in advance:


![image](https://github.com/emilk/egui/assets/1148717/53b0dda7-14c9-43be-a073-ad49865e69a6)

crates/egui/src/containers/popup.rs

@@ -307,7 +307,7 @@ pub fn popup_below_widget<R>(
 ///
 /// Useful for drop-down menus (combo boxes) or suggestion menus under text fields.
 ///
-/// The opened popup will have the same width as the parent.
+/// The opened popup will have a minimum width matching its parent.
 ///
 /// You must open the popup with [`Memory::open_popup`] or  [`Memory::toggle_popup`].
 ///
@@ -341,18 +341,21 @@ pub fn popup_above_or_below_widget<R>(
             AboveOrBelow::Below => (widget_response.rect.left_bottom(), Align2::LEFT_TOP),
         };
 
+        let frame = Frame::popup(parent_ui.style());
+        let frame_margin = frame.total_margin();
+        let inner_width = widget_response.rect.width() - frame_margin.sum().x;
+
         let inner = Area::new(popup_id)
             .order(Order::Foreground)
             .constrain(true)
             .fixed_pos(pos)
+            .default_width(inner_width)
             .pivot(pivot)
             .show(parent_ui.ctx(), |ui| {
-                let frame = Frame::popup(parent_ui.style());
-                let frame_margin = frame.total_margin();
                 frame
                     .show(ui, |ui| {
                         ui.with_layout(Layout::top_down_justified(Align::LEFT), |ui| {
-                            ui.set_width(widget_response.rect.width() - frame_margin.sum().x);
+                            ui.set_min_width(inner_width);
                             add_contents(ui)
                         })
                         .inner

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -172,8 +172,6 @@ impl WidgetGallery {
         egui::ComboBox::from_label("Take your pick")
             .selected_text(format!("{radio:?}"))
             .show_ui(ui, |ui| {
-                ui.style_mut().wrap_mode = Some(egui::TextWrapMode::Extend);
-                ui.set_min_width(60.0);
                 ui.selectable_value(radio, Enum::First, "First");
                 ui.selectable_value(radio, Enum::Second, "Second");
                 ui.selectable_value(radio, Enum::Third, "Third");

tests/test_size_pass/src/main.rs

@@ -9,6 +9,12 @@ fn main() -> eframe::Result<()> {
     let options = eframe::NativeOptions::default();
     eframe::run_simple_native("My egui App", options, move |ctx, _frame| {
         egui::CentralPanel::default().show(ctx, |ui| {
+            if ui.button("Reset egui memory").clicked() {
+                ctx.memory_mut(|mem| *mem = Default::default());
+            }
+
+            ui.separator();
+
             ui.label("The menu should be as wide as the widest button");
             ui.menu_button("Click for menu", |ui| {
                 let _ = ui.button("Narrow").clicked();
@@ -20,6 +26,23 @@ fn main() -> eframe::Result<()> {
                 ui.label("A separator:");
                 ui.separator();
             });
+
+            ui.separator();
+
+            let alternatives = [
+                "Short",
+                "Min",
+                "Very very long text that will extend",
+                "Short",
+            ];
+            let mut selected = 1;
+
+            egui::ComboBox::from_label("ComboBox").show_index(
+                ui,
+                &mut selected,
+                alternatives.len(),
+                |i| alternatives[i],
+            );
         });
     })
 }