CentralPanel overflows in a window unless there is a Top, Bottom or Right pannel

If a CentralPanel is added to a window (with or without a LeftPanel), it seems to be ignored by the window layout and overflows. Adding a Top, Bottom or Right pannel makes the issue go away.

**Screenshots**

**Expected behavior:** Window correctly wraps around central pannel and includes it in the layout. Window has no overflow.

**Desktop (please complete the following information):**
- OS: Ubuntu
- Version: tested on master
- Using eframe to run natively.

Code:

```rust
egui::Window::new("test").show(egui_context, |ui| {
// uncomment to make the issue go away

// egui::TopBottomPanel::top("top_panel").show_inside(ui, |ui| {
// egui::ScrollArea::vertical().show(ui, |ui| {
// lorem_ipsum(ui);
// });
// });

egui::SidePanel::left("left_panel").show_inside(ui, |ui| {
egui::ScrollArea::vertical().show(ui, |ui| {
lorem_ipsum(ui);
});
});

CentralPanel::default().show_inside(ui, |ui| {
ui.label("This guy is thinking outside the box for some reason")
})
});
```

Reproduce the issue with the provided egui::Window::show example, including the LeftPanel and CentralPanel but no Top, Bottom, or Right panel. Start by tracing Window::show, SidePanel::show_inside, and CentralPanel::show_inside, then verify that the window wraps the central panel without overflow.

Written by the indexing model from the issue text.