Window remains opaque when using WindowConfig::with_transparent(true)

[Loyreau]

Description

I‘m trying to use WindowConfig to set the background of the window to transparent with with_transparent(true)，but I found this option is not working at all.

After checking the layout with the Inspector, I noticed that the root view named "Window" is using a default theme which has a property setting the background color to black.

Expected Behavior

I think it would be better if this color option could switch to transparent automatically when a transparent configuration is applied to the window.

---

Environment

The dependency I'm using is pulled directly from the main branch at commit 348b02c which includes the recent PR #1059

Dependencies:
floem = { git = "https://github.com/lapce/floem.git", branch = "main" }

Code:

use floem::peniko::{Brush, Color};
use floem::prelude::Decorators;
use floem::views::{Button, Label, Stack};
use floem::window::{WinOsBackdropType, WindowConfig, WindowId};
use floem::{Application, IntoView};
use std::str::FromStr;

fn app_view(window_id: WindowId) -> impl IntoView {
    Stack::vertical((
        Label::new("Hello, Floem!"),
        Button::new("Inspector")
            .action(floem::action::inspect)
    ))
        .style(|s| s
            .padding_top(32.)
            .background(Brush::Solid(Color::from_str("#ff980099").unwrap()))
            .height_full()
        )
}

fn main() {
    let app = Application::new();
    let window_config = WindowConfig::default()
        .with_transparent(true)
        .show_titlebar(false)
        .with_win_os_config(|config| config
            .set_system_backdrop(WinOsBackdropType::TransientWindow)
        )
        .with_mac_os_config(|config| config
            .movable_by_window_background(true)
            .transparent_title_bar(true)
            .full_size_content_view(true)
            .hide_title(true)
        );
    app.window(app_view, Some(window_config)).run();
}

[Loyreau]

Im trying to figure out what's going on. And I realize it seems that its not because the window background is setting to theme bg color. I tried to set color to figure out which layer makes this problem.

I suddenly found that set the alpha_mode to wgpu::CompositeAlphaMode::PreMultiplied could solve this problem. although i dont know why and how. but this actually solve this problem temporarily.

on macOS, it only works when alpha_mode set to PostMultiplied.

But it should work normally because the default value of alpha_mode is Auto.

[jrmoulton]

We can make that configurable. A pr would be accepted. If you're not in a rush I'm currently rebuilding all of rendering and this will be easier to control once that lands