Deprecate CentralPanel::show (#7783)

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

Use `show_inside` instead, with a `Ui` instead of a `Context`

crates/egui/src/containers/panel.rs

@@ -1044,6 +1044,7 @@ impl CentralPanel {
     }
 
     /// Show the panel at the top level.
+    #[deprecated = "Use show_inside() instead"]
     pub fn show<R>(
         self,
         ctx: &Context,

crates/egui/src/context.rs

@@ -19,8 +19,8 @@ use crate::{
     ImmediateViewportRendererCallback, Key, KeyboardShortcut, Label, LayerId, Memory,
     ModifierNames, Modifiers, NumExt as _, Order, Painter, RawInput, Response, RichText,
     SafeAreaInsets, ScrollArea, Sense, Style, TextStyle, TextureHandle, TextureOptions, Ui,
-    ViewportBuilder, ViewportCommand, ViewportId, ViewportIdMap, ViewportIdPair, ViewportIdSet,
-    ViewportOutput, Visuals, Widget as _, WidgetRect, WidgetText,
+    UiBuilder, ViewportBuilder, ViewportCommand, ViewportId, ViewportIdMap, ViewportIdPair,
+    ViewportIdSet, ViewportOutput, Visuals, Widget as _, WidgetRect, WidgetText,
     animation_manager::AnimationManager,
     containers::{self, area::AreaState},
     data::output::PlatformOutput,
@@ -793,11 +793,23 @@ impl Context {
         let plugins = self.read(|ctx| ctx.plugins.ordered_plugins());
         #[expect(deprecated)]
         self.run(new_input, |ctx| {
-            crate::CentralPanel::no_frame().show(ctx, |ui| {
-                plugins.on_begin_pass(ui);
-                run_ui(ui);
-                plugins.on_end_pass(ui);
-            });
+            let mut top_ui = Ui::new(
+                ctx.clone(),
+                Id::new((ctx.viewport_id(), "__top_ui")),
+                UiBuilder::new()
+                    .layer_id(LayerId::background())
+                    .max_rect(ctx.globally_available_rect().round_ui()),
+            );
+
+            {
+                plugins.on_begin_pass(&mut top_ui);
+                run_ui(&mut top_ui);
+                plugins.on_end_pass(&mut top_ui);
+            }
+
+            // Inform ctx about what we actually used, so we can shrink the native window to fit.
+            // TODO(emilk): make better use of this somehow
+            ctx.pass_state_mut(|state| state.allocate_central_panel(top_ui.min_rect()));
         })
     }
 
@@ -3628,7 +3640,7 @@ impl Context {
     /// If AccessKit support is active for the current frame, get or create
     /// a node builder with the specified ID and return a mutable reference to it.
     /// For newly created nodes, the parent is the parent [`Ui`]s ID.
-    /// And an [`Ui`]s parent can be set with [`crate::UiBuilder::accessibility_parent`].
+    /// And an [`Ui`]s parent can be set with [`UiBuilder::accessibility_parent`].
     ///
     /// The `Context` lock is held while the given closure is called!
     ///

crates/egui_demo_lib/benches/benchmark.rs

@@ -140,19 +140,19 @@ pub fn criterion_benchmark(c: &mut Criterion) {
         let ctx = egui::Context::default();
         ctx.begin_pass(RawInput::default());
 
-        egui::CentralPanel::default().show(&ctx, |ui| {
-            c.bench_function("Painter::rect", |b| {
-                let painter = ui.painter();
-                let rect = ui.max_rect();
-                b.iter(|| {
-                    painter.rect(
-                        rect,
-                        2.0,
-                        egui::Color32::RED,
-                        (1.0, egui::Color32::WHITE),
-                        egui::StrokeKind::Inside,
-                    );
-                });
+        let painter =
+            egui::Painter::new(ctx.clone(), egui::LayerId::background(), ctx.content_rect());
+
+        c.bench_function("Painter::rect", |b| {
+            let rect = painter.clip_rect();
+            b.iter(|| {
+                painter.rect(
+                    rect,
+                    2.0,
+                    egui::Color32::RED,
+                    (1.0, egui::Color32::WHITE),
+                    egui::StrokeKind::Inside,
+                );
             });
         });
 

crates/egui_kittest/tests/accesskit.rs

@@ -18,8 +18,8 @@ fn empty_ui_should_return_tree_with_only_root_window() {
 
     assert_eq!(
         output.nodes.len(),
-        4,
-        "Expected the root node and two Uis and a Frame for the panel"
+        2,
+        "Expected the root node and the top level Ui; found: {output:#?}",
     );
 
     assert_eq!(
@@ -28,8 +28,8 @@ fn empty_ui_should_return_tree_with_only_root_window() {
             .iter()
             .filter(|(_, n)| n.role() == Role::GenericContainer)
             .count(),
-        3,
-        "Expected two Uis and one Frame as GenericContainer nodes.",
+        1,
+        "Expected a single Ui as a GenericContainer node.",
     );
 
     let (id, root) = &output.nodes[0];

examples/custom_window_frame/src/main.rs

@@ -32,7 +32,7 @@ impl eframe::App for MyApp {
     }
 
     fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
-        custom_window_frame(ui.ctx(), "egui with custom frame", |ui| {
+        custom_window_frame(ui, "egui with custom frame", |ui| {
             ui.label("This is just the contents of the window.");
             ui.horizontal(|ui| {
                 ui.label("egui theme:");
@@ -42,18 +42,20 @@ impl eframe::App for MyApp {
     }
 }
 
-fn custom_window_frame(ctx: &egui::Context, title: &str, add_contents: impl FnOnce(&mut egui::Ui)) {
-    use egui::{CentralPanel, UiBuilder};
+fn custom_window_frame(ui: &mut egui::Ui, title: &str, add_contents: impl FnOnce(&mut egui::Ui)) {
+    use egui::UiBuilder;
 
     let panel_frame = egui::Frame::new()
-        .fill(ctx.global_style().visuals.window_fill())
+        .fill(ui.global_style().visuals.window_fill())
         .corner_radius(10)
-        .stroke(ctx.global_style().visuals.widgets.noninteractive.fg_stroke)
+        .stroke(ui.global_style().visuals.widgets.noninteractive.fg_stroke)
         .outer_margin(1); // so the stroke is within the bounds
 
-    CentralPanel::default().frame(panel_frame).show(ctx, |ui| {
+    panel_frame.show(ui, |ui| {
         let app_rect = ui.max_rect();
 
+        ui.expand_to_include_rect(app_rect); // Expand frame to include it all
+
         let title_bar_height = 32.0;
         let title_bar_rect = {
             let mut rect = app_rect;