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)

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],
+            );
         });
     })
 }