Add hook to tap into window creation similar to EventLoopBuilder hook in eframe

[twop]

Problem

I would love to customize appearance beyond what is available in eframe::NativeOptions (example below is on macOS).

Specifically I would love to achieve this look (top window, code snippet is taken from winit):

It is a combination of these winit settings:

    let event_loop = EventLoop::new();

    let window_1 = WindowBuilder::new()
        .with_titlebar_buttons_hidden(true)
        .with_title_hidden(true)
        .with_titlebar_transparent(true)
        .build(&event_loop);

Note that these methods are specific to macOS (defined in WindowBuilderExtMacOS).
The closest that eframe provides today is

 eframe::NativeOptions {
        fullsize_content: true,
        decorated: false
}

But it removes the ability to resize and "roundness" of the window.

Desired solution

I would love to to tap into WindowBuilder with a hook similar to EventLoopBuilder.
Something like that

let options = eframe::NativeOptions {
    // proposed API for window builder
    window_builder: Some(Box::new(|builder| {
        #[cfg(target_os = "macos")]
        {
            use winit::platform::macos::WindowBuilderExtMacOS;
            builder
                .with_titlebar_buttons_hidden(true)
                .with_title_hidden(true)
                .with_titlebar_transparent(true)
        }
    })),

    // existing hook for event loop
    event_loop_builder: Some(Box::new(|builder| {
        #[cfg(target_os = "macos")]
        {
            use winit::platform::macos::{ActivationPolicy, EventLoopBuilderExtMacOS};
            builder.with_activation_policy(ActivationPolicy::Accessory);
        }
    })),

    ..Default::default()
};

Alternative

Mimic winit window builder API in eframe::NativeOptions. It feels less desirable to me because

1. There is already a precedent for EventLoop hook
2. It seems that eframe::NativeOptions is mostly 1:1 to winit options, thus it feels repetitive.
3. With little code in eframe this provides access to a lot of features, AND it is future proof, e.g. when winit adds features eframe gets these features for "free".

---

Happy to contribute the code

[twop]

I have a fork when it is implemented https://github.com/twop/egui/tree/focus-and-window-builder
Here is the gist:

and here is the usage of it

Should I submit a PR?

[bates64]

Should I submit a PR?

Placing my hat in the ring for this - yes!

This would be a breaking change because NativeOptions isn't marked exhaustive, the PR should probably mark it as such to make it non-breaking to add new options in the future.

[twop]

created a PR just in case #3390