Combo boxes has a scroll area

Closes https://github.com/emilk/egui/issues/75

CHANGELOG.md

@@ -30,6 +30,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * Made some more fields of `RawInput` optional.
 * `Slider` and `DragValue` uses fewer decimals by default. See the full precision by hovering over the value.
 * `egui::App`: added `fn name(&self)` and `fn clear_color(&self)`.
+* Combo boxes has scroll bars when needed.
 
 ### Fixed 🐛
 

egui/src/containers/combo_box.rs

@@ -23,6 +23,8 @@ pub fn combo_box(
     selected: impl Into<Label>,
     menu_contents: impl FnOnce(&mut Ui),
 ) -> Response {
+    const MAX_COMBO_HEIGHT: f32 = 128.0;
+
     let popup_id = button_id.with("popup");
     let selected = selected.into();
 
@@ -61,10 +63,12 @@ pub fn combo_box(
                 let frame_margin = frame.margin;
                 frame.show(ui, |ui| {
                     ui.with_layout(Layout::top_down_justified(Align::left()), |ui| {
-                        ui.set_min_width(button_response.rect.width() - 2.0 * frame_margin.x);
-                        menu_contents(ui);
+                        let width = button_response.rect.width() - 2.0 * frame_margin.x;
+                        ui.set_min_width(width);
+                        ui.set_max_width(width);
+                        ScrollArea::from_max_height(MAX_COMBO_HEIGHT).show(ui, menu_contents);
+                    });
                 });
-                })
             });
 
         if ui.input().key_pressed(Key::Escape) || ui.input().mouse.click && !button_response.clicked

egui/src/containers/scroll_area.rs

@@ -102,13 +102,15 @@ impl ScrollArea {
             max_scroll_bar_width * ui.ctx().animate_bool(id, state.show_scroll)
         };
 
+        let available_outer = ui.available_rect_before_wrap();
+
         let outer_size = vec2(
-            ui.available_width(),
-            ui.available_size_before_wrap().y.at_most(max_height),
+            available_outer.width(),
+            available_outer.height().at_most(max_height),
         );
 
         let inner_size = outer_size - vec2(current_scroll_bar_width, 0.0);
-        let inner_rect = Rect::from_min_size(ui.available_rect_before_wrap().min, inner_size);
+        let inner_rect = Rect::from_min_size(available_outer.min, inner_size);
 
         let mut content_ui = ui.child_ui(
             Rect::from_min_size(
@@ -153,13 +155,14 @@ impl Prepared {
 
         let content_size = content_ui.min_size();
 
-        let inner_rect = Rect::from_min_size(
-            inner_rect.min,
-            vec2(
-                inner_rect.width().max(content_size.x), // Expand width to fit content
-                inner_rect.height(),
-            ),
-        );
+        let width = if inner_rect.width().is_finite() {
+            inner_rect.width().max(content_size.x) // Expand width to fit content
+        } else {
+            // ScrollArea is in an infinitely wide parent
+            content_size.x
+        };
+
+        let inner_rect = Rect::from_min_size(inner_rect.min, vec2(width, inner_rect.height()));
 
         let outer_rect = Rect::from_min_size(
             inner_rect.min,