improv: remove double coloring of content_container windows

This sets the main content and the header bar to transparent when `content_container` is true, so that things aren't colored twice and overlayed on top of each other.
This ensures that modifying color alpha behaves as expected, especially for frosted glass.

src/app/mod.rs

@@ -689,7 +689,6 @@ impl<App: Application> ApplicationExt for App {
                 .apply(container)
                 .width(iced::Length::Fill)
                 .height(iced::Length::Fill)
-                .class(crate::theme::Container::WindowBackground)
                 .apply(|w| id_container(w, iced_core::id::Id::new("COSMIC_content_container")))
                 .into()
         } else {
@@ -713,6 +712,7 @@ impl<App: Application> ApplicationExt for App {
                         .focused(focused)
                         .maximized(maximized)
                         .sharp_corners(sharp_corners)
+                        .transparent(content_container)
                         .title(&core.window.header_title)
                         .on_drag(crate::Action::Cosmic(Action::Drag))
                         .on_right_click(crate::Action::Cosmic(Action::ShowWindowMenu))

src/theme/style/iced.rs

@@ -396,6 +396,7 @@ pub enum Container<'a> {
     HeaderBar {
         focused: bool,
         sharp_corners: bool,
+        transparent: bool,
     },
     List,
     Primary,
@@ -511,6 +512,7 @@ impl iced_container::Catalog for Theme {
             Container::HeaderBar {
                 focused,
                 sharp_corners,
+                transparent,
             } => {
                 let (icon_color, text_color) = if *focused {
                     (
@@ -527,7 +529,11 @@ impl iced_container::Catalog for Theme {
                 iced_container::Style {
                     icon_color: Some(icon_color),
                     text_color: Some(text_color),
-                    background: Some(iced::Background::Color(cosmic.background.base.into())),
+                    background: if *transparent {
+                        None
+                    } else {
+                        Some(iced::Background::Color(cosmic.background.base.into()))
+                    },
                     border: Border {
                         radius: [
                             if *sharp_corners {

src/widget/header_bar.rs

@@ -28,6 +28,7 @@ pub fn header_bar<'a, Message>() -> HeaderBar<'a, Message> {
         is_ssd: false,
         on_double_click: None,
         is_condensed: false,
+        transparent: false,
     }
 }
 
@@ -92,6 +93,9 @@ pub struct HeaderBar<'a, Message> {
 
     /// Whether the headerbar should be compact
     is_condensed: bool,
+
+    /// Whether the headerbar should be transparent
+    transparent: bool,
 }
 
 impl<'a, Message: Clone + 'static> HeaderBar<'a, Message> {
@@ -412,6 +416,7 @@ impl<'a, Message: Clone + 'static> HeaderBar<'a, Message> {
             .class(crate::theme::Container::HeaderBar {
                 focused: self.focused,
                 sharp_corners: self.sharp_corners,
+                transparent: self.transparent,
             })
             .center_y(Length::Shrink)
             .apply(widget::mouse_area);