Opt-in to not store some selected state when using persistence

```rust
egui::Window::new("code")
.open(&mut self.open)
.scroll(true)
.resizable(true)
.default_height(10.0)
.show(ctx, |ui| {
ui.label(
"This window is resizable and has a scroll area. You can shrink it to any size",
);
ui.separator();
ui.code(contents);
});
```

this is basically the same setup as [in the demo app](https://github.com/emilk/egui/blob/52e3663958cd8aa669c55ab077e5bf46dc0769db/egui_demo_lib/src/apps/demo/tests.rs#L354), but the window seems to follow only the size of its contents, expanding off screen for long texts. Am I missing something or did `Window::default_height` break in a recent patch?

Reproduce the snippet and compare it with egui_demo_lib/src/apps/demo/tests.rs around the linked example. Then trace the Window sizing behavior, including default_height, and clarify whether persistence is involved. Done means the reported behavior is explained and the requested change or regression test is identified.

Written by the indexing model from the issue text.