Allow the creation of a Window in a not focused state

[Selene-Amanita]

Objective

Allow creating a new window without it being focused, when Window's focused is `false.

Solution

Use winit's WindowBuilder's with_active method

Notes

• winit's doc lists redox's Orbital as an unsupported platform, but since Bevy doesn't officially support this platform, I didn't put it in the documentation.
• I only tested on Linux, which is an unsupported platform. I can give you a test code if you want to test on another platform.
• I initially put a line here to set the Bevy Window's focused to winit_window.has_focus() after window creation to avoid the case where with_active is not supported, the window is spawned focused, no WindowFocused event is triggered, and Bevy Window would be desynced from winit's window. But after testing on Linux (which doesn't support with_active) it seems like at that point has_focus returns false and the event is triggered, so I removed it. Do you think I should add it back to be safe?

Changelog

• A new unfocused Window can be created by setting focused to false.

Migration Guide

• If a Window is spawned with focused set to false, it will now start not focused on supported platforms.

[mockersf]

is it just at spawn time that it's not possible? can the window be set as unfocused?

[Selene-Amanita]

Thanks I didn't think about checking that.

Setting the window focused through code is not possible for "iOS / Android / Web / Wayland" so a different subset of platforms. I'll commit that later. https://docs.rs/winit/latest/winit/window/struct.Window.html#method.focus_window

Also I think it can never be set unfocused through code, here in Bevy's code:

            if window.focused != cache.window.focused && window.focused {
                winit_window.focus_window();
            }

If set to false later it will just be ignored? I need to mention that too.

What I wrote is only at spawn time yes: https://docs.rs/winit/latest/winit/window/struct.WindowBuilder.html#method.with_active

Maybe I can put a link to winit's doc for those two methods here, so people can check if Bevy's doc is outdated and more platforms are supported some day?

[Selene-Amanita]

I clarified that in this commit: 1e61bb6

Note that I think (didn't test to verify) that if it is set (from focused) to false, or set (from unfocused) to true on an unsupported platform, it will just be desynced. I can try to fix that in this PR but I think it would be better in a follow-up PR.