Constrain demo windows to the space left after the panels (#7785)

crates/egui_demo_lib/src/demo/about.rs

@@ -8,14 +8,15 @@ impl crate::Demo for About {
         "About egui"
     }
 
-    fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
+    fn show(&mut self, ui: &mut egui::Ui, open: &mut bool) {
         egui::Window::new(self.name())
             .default_width(320.0)
             .default_height(480.0)
             .open(open)
             .resizable([true, false])
             .scroll(false)
-            .show(ctx, |ui| {
+            .constrain_to(ui.available_rect_before_wrap())
+            .show(ui, |ui| {
                 use crate::View as _;
                 self.ui(ui);
             });