Remove some uses of top-level panels in our examples (#7729)

We're phasing out top-level panels (panels that use `Context` directly,
instead of being inside another `Ui`).
As a first step, stop using them in our demo library and application.

* Part of https://github.com/emilk/egui/issues/3524

crates/egui_demo_lib/src/demo/demo_app_windows.rs

@@ -195,11 +195,11 @@ impl Default for DemoWindows {
 
 impl DemoWindows {
     /// Show the app ui (menu bar and windows).
-    pub fn ui(&mut self, ctx: &Context) {
-        if is_mobile(ctx) {
-            self.mobile_ui(ctx);
+    pub fn ui(&mut self, ui: &mut egui::Ui) {
+        if is_mobile(ui.ctx()) {
+            self.mobile_ui(ui);
         } else {
-            self.desktop_ui(ctx);
+            self.desktop_ui(ui);
         }
     }
 
@@ -207,36 +207,36 @@ impl DemoWindows {
         self.open.contains(About::default().name())
     }
 
-    fn mobile_ui(&mut self, ctx: &Context) {
+    fn mobile_ui(&mut self, ui: &mut egui::Ui) {
         if self.about_is_open() {
             let mut close = false;
-            egui::CentralPanel::default().show(ctx, |ui| {
-                egui::ScrollArea::vertical()
-                    .auto_shrink(false)
-                    .show(ui, |ui| {
-                        self.groups.about.ui(ui);
-                        ui.add_space(12.0);
-                        ui.vertical_centered_justified(|ui| {
-                            if ui
-                                .button(egui::RichText::new("Continue to the demo!").size(20.0))
-                                .clicked()
-                            {
-                                close = true;
-                            }
-                        });
+
+            egui::ScrollArea::vertical()
+                .auto_shrink(false)
+                .show(ui, |ui| {
+                    self.groups.about.ui(ui);
+                    ui.add_space(12.0);
+                    ui.vertical_centered_justified(|ui| {
+                        if ui
+                            .button(egui::RichText::new("Continue to the demo!").size(20.0))
+                            .clicked()
+                        {
+                            close = true;
+                        }
                     });
-            });
+                });
+
             if close {
                 set_open(&mut self.open, About::default().name(), false);
             }
         } else {
-            self.mobile_top_bar(ctx);
-            self.groups.windows(ctx, &mut self.open);
+            self.mobile_top_bar(ui);
+            self.groups.windows(ui.ctx(), &mut self.open);
         }
     }
 
-    fn mobile_top_bar(&mut self, ctx: &Context) {
-        egui::Panel::top("menu_bar").show(ctx, |ui| {
+    fn mobile_top_bar(&mut self, ui: &mut egui::Ui) {
+        egui::Panel::top("menu_bar").show_inside(ui, |ui| {
             menu::MenuBar::new()
                 .config(menu::MenuConfig::new().style(StyleModifier::default()))
                 .ui(ui, |ui| {
@@ -261,12 +261,12 @@ impl DemoWindows {
         });
     }
 
-    fn desktop_ui(&mut self, ctx: &Context) {
+    fn desktop_ui(&mut self, ui: &mut egui::Ui) {
         egui::Panel::right("egui_demo_panel")
             .resizable(false)
             .default_size(160.0)
             .min_size(160.0)
-            .show(ctx, |ui| {
+            .show_inside(ui, |ui| {
                 ui.add_space(4.0);
                 ui.vertical_centered(|ui| {
                     ui.heading("✒ egui demos");
@@ -289,13 +289,13 @@ impl DemoWindows {
                 self.demo_list_ui(ui);
             });
 
-        egui::Panel::top("menu_bar").show(ctx, |ui| {
+        egui::Panel::top("menu_bar").show_inside(ui, |ui| {
             menu::MenuBar::new().ui(ui, |ui| {
                 file_menu_button(ui);
             });
         });
 
-        self.groups.windows(ctx, &mut self.open);
+        self.groups.windows(ui.ctx(), &mut self.open);
     }
 
     fn demo_list_ui(&mut self, ui: &mut egui::Ui) {

crates/egui_demo_lib/src/demo/panels.rs

@@ -72,6 +72,7 @@ impl crate::View for Panels {
                 });
             });
 
+        // TODO(emilk): This extra panel is superfluous - just use what's left of `ui` instead
         egui::CentralPanel::default().show_inside(ui, |ui| {
             ui.vertical_centered(|ui| {
                 ui.heading("Central Panel");

crates/egui_demo_lib/src/easy_mark/easy_mark_editor.rs

@@ -32,15 +32,15 @@ impl Default for EasyMarkEditor {
 }
 
 impl EasyMarkEditor {
-    pub fn panels(&mut self, ctx: &egui::Context) {
-        egui::Panel::bottom("easy_mark_bottom").show(ctx, |ui| {
+    pub fn panels(&mut self, ui: &mut egui::Ui) {
+        egui::Panel::bottom("easy_mark_bottom").show_inside(ui, |ui| {
             let layout = egui::Layout::top_down(egui::Align::Center).with_main_justify(true);
             ui.allocate_ui_with_layout(ui.available_size(), layout, |ui| {
                 ui.add(crate::egui_github_link_file!())
             })
         });
 
-        egui::CentralPanel::default().show(ctx, |ui| {
+        egui::CentralPanel::default().show_inside(ui, |ui| {
             self.ui(ui);
         });
     }

crates/egui_demo_lib/src/lib.rs

@@ -74,7 +74,9 @@ fn test_egui_e2e() {
     const NUM_FRAMES: usize = 5;
     for _ in 0..NUM_FRAMES {
         let full_output = ctx.run(raw_input.clone(), |ctx| {
-            demo_windows.ui(ctx);
+            egui::CentralPanel::default().show(ctx, |ui| {
+                demo_windows.ui(ui);
+            });
         });
         let clipped_primitives = ctx.tessellate(full_output.shapes, full_output.pixels_per_point);
         assert!(!clipped_primitives.is_empty());
@@ -93,7 +95,9 @@ fn test_egui_zero_window_size() {
     const NUM_FRAMES: usize = 5;
     for _ in 0..NUM_FRAMES {
         let full_output = ctx.run(raw_input.clone(), |ctx| {
-            demo_windows.ui(ctx);
+            egui::CentralPanel::default().show(ctx, |ui| {
+                demo_windows.ui(ui);
+            });
         });
         let clipped_primitives = ctx.tessellate(full_output.shapes, full_output.pixels_per_point);
         assert!(