eframe: Don't show window until after initialization (#2279)

* Don't show window until after initialization

Shortens #1802, but does not completely solve it

* format code

* Present first frame immediately before showing window

This resolves the white flash almost completely, but is a hack. Window
visibility should be derived from the AppOutput, and the first frame
should not be painted before the event loop has processed initial
events.

Working on a better implementation.

* Integrate window showing with AppOutput

This allows an app to keep the window hidden (never shown) by calling
Frame.set_visible(false) on the first update. This includes a slightly
less nasty hack than the last commit did.

Also fixes an accidental cross-contamination of pull requests.

* fmt

* add comments

* add comments

* add comments

* add comments

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

crates/eframe/src/native/epi_integration.rs

@@ -146,7 +146,7 @@ pub fn handle_app_output(
         fullscreen,
         drag_window,
         window_pos,
-        visible,
+        visible: _, // handled in post_present
         always_on_top,
     } = app_output;
 
@@ -183,10 +183,6 @@ pub fn handle_app_output(
         let _ = window.drag_window();
     }
 
-    if let Some(visible) = visible {
-        window.set_visible(visible);
-    }
-
     if let Some(always_on_top) = always_on_top {
         window.set_always_on_top(always_on_top);
     }
@@ -240,7 +236,10 @@ impl EpiIntegration {
                 native_pixels_per_point: Some(native_pixels_per_point),
                 window_info: read_window_info(window, egui_ctx.pixels_per_point()),
             },
-            output: Default::default(),
+            output: epi::backend::AppOutput {
+                visible: Some(true),
+                ..Default::default()
+            },
             storage,
             #[cfg(feature = "glow")]
             gl,
@@ -325,6 +324,7 @@ impl EpiIntegration {
             if app_output.close {
                 self.close = app.on_close_event();
             }
+            self.frame.output.visible = app_output.visible; // this is handled by post_present
             handle_app_output(window, self.egui_ctx.pixels_per_point(), app_output);
         }
 
@@ -341,6 +341,12 @@ impl EpiIntegration {
         app.post_rendering(window_size_px, &self.frame);
     }
 
+    pub fn post_present(&mut self, window: &winit::window::Window) {
+        if let Some(visible) = self.frame.output.visible.take() {
+            window.set_visible(visible);
+        }
+    }
+
     pub fn handle_platform_output(
         &mut self,
         window: &winit::window::Window,

crates/eframe/src/native/run.rs

@@ -349,8 +349,9 @@ mod glow_integration {
             };
             let window_settings = epi_integration::load_window_settings(storage);
 
-            let window_builder =
+            let window_builder = epi_integration::window_builder(native_options, &window_settings)
-                epi_integration::window_builder(native_options, &window_settings).with_title(title);
+                .with_title(title)
+                .with_visible(false); // Keep hidden until we've painted something. See https://github.com/emilk/egui/pull/2279
 
             let gl_window = unsafe {
                 glutin::ContextBuilder::new()
@@ -512,6 +513,8 @@ mod glow_integration {
                     gl_window.swap_buffers().unwrap();
                 }
 
+                integration.post_present(window);
+
                 let control_flow = if integration.should_close() {
                     EventResult::Exit
                 } else if repaint_after.is_zero() {
@@ -733,6 +736,7 @@ mod wgpu_integration {
             let window_settings = epi_integration::load_window_settings(storage);
             epi_integration::window_builder(native_options, &window_settings)
                 .with_title(title)
+                .with_visible(false) // Keep hidden until we've painted something. See https://github.com/emilk/egui/pull/2279
                 .build(event_loop)
                 .unwrap()
         }
@@ -891,6 +895,7 @@ mod wgpu_integration {
                 );
 
                 integration.post_rendering(app.as_mut(), window);
+                integration.post_present(window);
 
                 let control_flow = if integration.should_close() {
                     EventResult::Exit