Layout with a few items inside Window blocks resizing

[TicClick]

suppose I have the following layout:

pub enum Tab {
  First,
  Second,
}

let mut tab = Tab::First;
egui::Window::new("window").open(true).show(ctx, |ui| {
  ui.horizontal(|ui| {
    ui.vertical(|ui| {
      ui.selectable_value(&mut tab, Tab::First, "First");
      ui.selectable_value(&mut tab, Tab::Second, "Second");
    });
    ui.separator();

    egui::ScrollArea::vertical().auto_shrink([false; 2]).show(ui, |ui| {
      ui.vertical(|ui| {
        for s in 0..5 {
          ui.label(s.to_string());
        }
      });
    });
  });
});

visualized:

|---------------------------------|
| first  | 0                      |
| second | 1                      |
|---------------------------------|

(2, 3, 4 are invisible but may be scrolled to)

I want the window to be of the following:

• non-resizable, but with sensible default height and width, OR
• resizable from the get go

it may be resized horizontally, but it shrinks to the minimum possible height, which depends on amount of selectable_values I use. I've tried adding ui.allocate_space(ui.available_space()) inside the first lambda of ui.vertical(), but oddly enough it makes the window bloat horizontally instead of vertically.

what are my options in this case?