Add possibility of panels inside UI (i.e. windows) (#624)

* Adding possibility to have panels inside UI

* Adding window with panels demo

egui_demo_lib/src/apps/demo/demo_app_windows.rs

@@ -28,6 +28,7 @@ impl Default for Demos {
             Box::new(super::widget_gallery::WidgetGallery::default()),
             Box::new(super::window_options::WindowOptions::default()),
             Box::new(super::tests::WindowResizeTest::default()),
+            Box::new(super::window_with_panels::WindowWithPanels::default()),
         ])
     }
 }

egui_demo_lib/src/apps/demo/mod.rs

@@ -21,6 +21,7 @@ pub mod tests;
 pub mod toggle_switch;
 pub mod widget_gallery;
 pub mod window_options;
+pub mod window_with_panels;
 
 pub use {
     app::DemoApp, demo_app_windows::DemoWindows, misc_demo_window::MiscDemoWindow,

egui_demo_lib/src/apps/demo/window_with_panels.rs

@@ -0,0 +1,89 @@
+use egui::{menu, Align, CentralPanel, Layout, ScrollArea, SidePanel, TopBottomPanel};
+
+#[derive(Clone, PartialEq, Default)]
+#[cfg_attr(feature = "persistence", derive(serde::Deserialize, serde::Serialize))]
+pub struct WindowWithPanels {}
+
+impl super::Demo for WindowWithPanels {
+    fn name(&self) -> &'static str {
+        "🗖 Window With Panels"
+    }
+    fn show(&mut self, ctx: &egui::CtxRef, open: &mut bool) {
+        use super::View;
+        let window = egui::Window::new("Window with Panels")
+            .scroll(false)
+            .title_bar(true)
+            .resizable(true)
+            .collapsible(false)
+            .enabled(true)
+            .open(open);
+        window.show(ctx, |ui| self.ui(ui));
+    }
+}
+
+impl super::View for WindowWithPanels {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        let left_panel_min_width = 100.;
+        let left_panel_max_width = left_panel_min_width * 4.;
+        let bottom_height = 25.;
+
+        ui.expand_to_include_rect(ui.max_rect()); // Expand frame to include it all
+
+        let mut top_rect = ui.available_rect_before_wrap_finite();
+        top_rect.min.y += ui.spacing().item_spacing.y;
+        let mut top_ui = ui.child_ui(top_rect, Layout::top_down(Align::Max));
+
+        let top_response = TopBottomPanel::top("window_menu")
+            .resizable(false)
+            .show_inside(&mut top_ui, |ui| {
+                menu::bar(ui, |ui| {
+                    menu::menu(ui, "Menu", |ui| {
+                        if ui.button("Option 1").clicked() {}
+                        if ui.button("Option 2").clicked() {}
+                        if ui.button("Option 3").clicked() {}
+                    });
+                });
+            });
+
+        let mut left_rect = ui.available_rect_before_wrap_finite();
+        left_rect.min.y = top_response.response.rect.max.y + ui.spacing().item_spacing.y;
+        let mut left_ui = ui.child_ui(left_rect, Layout::top_down(Align::Max));
+
+        let left_response = SidePanel::left("Folders")
+            .resizable(true)
+            .min_width(left_panel_min_width)
+            .max_width(left_panel_max_width)
+            .show_inside(&mut left_ui, |ui| {
+                ScrollArea::auto_sized().show(ui, |ui| {
+                    ui.vertical(|ui| {
+                        ui.label("Left Panel");
+                    })
+                })
+            });
+
+        let mut right_rect = ui.available_rect_before_wrap_finite();
+        right_rect.min.x = left_response.response.rect.max.x;
+        right_rect.min.y = top_response.response.rect.max.y + ui.spacing().item_spacing.y;
+        let mut right_ui = ui.child_ui(right_rect, Layout::top_down(Align::Max));
+
+        CentralPanel::default().show_inside(&mut right_ui, |ui| {
+            let mut rect = ui.min_rect();
+            let mut bottom_rect = rect;
+            bottom_rect.min.y = ui.max_rect_finite().max.y - bottom_height;
+            rect.max.y = bottom_rect.min.y - ui.spacing().indent;
+            let mut child_ui = ui.child_ui(rect, Layout::top_down(Align::Min));
+            let mut bottom_ui = ui.child_ui(bottom_rect, Layout::bottom_up(Align::Max));
+            ScrollArea::auto_sized().show(&mut child_ui, |ui| {
+                ui.vertical(|ui| {
+                    ui.label("Central Panel");
+                })
+            });
+            bottom_ui.vertical(|ui| {
+                ui.separator();
+                ui.horizontal(|ui| {
+                    ui.label("Bottom Content");
+                });
+            });
+        });
+    }
+}