Follow the System Theme in egui (#4860)

* Some initial progress towards #4490

This PR just moves `Theme` and the "follow system theme" settings to
egui and adds `RawInput.system_theme`.
A follow-up PR can then introduce the two separate `dark_mode_style` and
`light_mode_style` fields on `Options`.


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### Breaking changes

The options `follow_system_theme` and `default_theme` has been moved
from `eframe` into `egui::Options`, settable with `ctx.options_mut`

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/native/epi_integration.rs

@@ -10,7 +10,7 @@ use raw_window_handle::{HasDisplayHandle as _, HasWindowHandle as _};
 use egui::{DeferredViewportUiCallback, NumExt as _, ViewportBuilder, ViewportId};
 use egui_winit::{EventResponse, WindowSettings};
 
-use crate::{epi, Theme};
+use crate::epi;
 
 pub fn viewport_builder(
     egui_zoom_factor: f32,
@@ -158,7 +158,6 @@ pub struct EpiIntegration {
     close: bool,
 
     can_drag_window: bool,
-    follow_system_theme: bool,
     #[cfg(feature = "persistence")]
     persist_window: bool,
     app_icon_setter: super::app_icon::AppTitleIconSetter,
@@ -169,7 +168,6 @@ impl EpiIntegration {
     pub fn new(
         egui_ctx: egui::Context,
         window: &winit::window::Window,
-        system_theme: Option<Theme>,
         app_name: &str,
         native_options: &crate::NativeOptions,
         storage: Option<Box<dyn epi::Storage>>,
@@ -180,10 +178,7 @@ impl EpiIntegration {
         #[cfg(feature = "wgpu")] wgpu_render_state: Option<egui_wgpu::RenderState>,
     ) -> Self {
         let frame = epi::Frame {
-            info: epi::IntegrationInfo {
-                system_theme,
-                cpu_usage: None,
-            },
+            info: epi::IntegrationInfo { cpu_usage: None },
             storage,
             #[cfg(feature = "glow")]
             gl,
@@ -217,7 +212,6 @@ impl EpiIntegration {
             pending_full_output: Default::default(),
             close: false,
             can_drag_window: false,
-            follow_system_theme: native_options.follow_system_theme,
             #[cfg(feature = "persistence")]
             persist_window: native_options.persist_window,
             app_icon_setter,
@@ -251,11 +245,6 @@ impl EpiIntegration {
                 state: ElementState::Pressed,
                 ..
             } => self.can_drag_window = true,
-            WindowEvent::ThemeChanged(winit_theme) if self.follow_system_theme => {
-                let theme = theme_from_winit_theme(*winit_theme);
-                self.frame.info.system_theme = Some(theme);
-                self.egui_ctx.set_visuals(theme.egui_visuals());
-            }
             _ => {}
         }
 
@@ -398,10 +387,3 @@ pub fn load_egui_memory(_storage: Option<&dyn epi::Storage>) -> Option<egui::Mem
     #[cfg(not(feature = "persistence"))]
     None
 }
-
-pub(crate) fn theme_from_winit_theme(theme: winit::window::Theme) -> Theme {
-    match theme {
-        winit::window::Theme::Dark => Theme::Dark,
-        winit::window::Theme::Light => Theme::Light,
-    }
-}

crates/eframe/src/native/glow_integration.rs

@@ -228,14 +228,11 @@ impl GlowWinitApp {
             }
         }
 
-        let system_theme =
-            winit_integration::system_theme(&glutin.window(ViewportId::ROOT), &self.native_options);
         let painter = Rc::new(RefCell::new(painter));
 
         let integration = EpiIntegration::new(
             egui_ctx,
             &glutin.window(ViewportId::ROOT),
-            system_theme,
             &self.app_name,
             &self.native_options,
             storage,
@@ -281,9 +278,6 @@ impl GlowWinitApp {
             }
         }
 
-        let theme = system_theme.unwrap_or(self.native_options.default_theme);
-        integration.egui_ctx.set_visuals(theme.egui_visuals());
-
         if self
             .native_options
             .viewport
@@ -1120,6 +1114,7 @@ impl GlutinWindowContext {
                 viewport_id,
                 event_loop,
                 Some(window.scale_factor() as f32),
+                window.theme(),
                 self.max_texture_side,
             )
         });

crates/eframe/src/native/wgpu_integration.rs

@@ -203,11 +203,9 @@ impl WgpuWinitApp {
 
         let wgpu_render_state = painter.render_state();
 
-        let system_theme = winit_integration::system_theme(&window, &self.native_options);
         let integration = EpiIntegration::new(
             egui_ctx.clone(),
             &window,
-            system_theme,
             &self.app_name,
             &self.native_options,
             storage,
@@ -243,6 +241,7 @@ impl WgpuWinitApp {
             ViewportId::ROOT,
             event_loop,
             Some(window.scale_factor() as f32),
+            window.theme(),
             painter.max_texture_side(),
         );
 
@@ -251,8 +250,6 @@ impl WgpuWinitApp {
             let event_loop_proxy = self.repaint_proxy.lock().clone();
             egui_winit.init_accesskit(&window, event_loop_proxy);
         }
-        let theme = system_theme.unwrap_or(self.native_options.default_theme);
-        egui_ctx.set_visuals(theme.egui_visuals());
 
         let app_creator = std::mem::take(&mut self.app_creator)
             .expect("Single-use AppCreator has unexpectedly already been taken");
@@ -872,6 +869,7 @@ impl Viewport {
                     viewport_id,
                     event_loop,
                     Some(window.scale_factor() as f32),
+                    window.theme(),
                     painter.max_texture_side(),
                 ));
 

crates/eframe/src/native/winit_integration.rs

@@ -118,16 +118,6 @@ pub enum EventResult {
     Exit,
 }
 
-pub fn system_theme(window: &Window, options: &crate::NativeOptions) -> Option<crate::Theme> {
-    if options.follow_system_theme {
-        window
-            .theme()
-            .map(super::epi_integration::theme_from_winit_theme)
-    } else {
-        None
-    }
-}
-
 #[cfg(feature = "accesskit")]
 pub(crate) fn on_accesskit_window_event(
     egui_winit: &mut egui_winit::State,