From 9b9894d8083df4a895a0a123fb534d2c9574e7b1 Mon Sep 17 00:00:00 2001
From: Vladyslav Batyrenko <mvlabat@gmail.com>
Date: Tue, 30 Jul 2024 23:36:33 +0300
Subject: [PATCH] Fix redraws (#293)

* Attempt to reproduce #292

* Fix repaint corner cases
---
 examples/side_panel.rs | 16 +++++++++++++++-
 src/systems.rs         | 21 +++++++++++++++++++--
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/examples/side_panel.rs b/examples/side_panel.rs
index 5989f27fc..14e9c56ef 100644
--- a/examples/side_panel.rs
+++ b/examples/side_panel.rs
@@ -1,4 +1,4 @@
-use bevy::{prelude::*, window::PrimaryWindow};
+use bevy::{prelude::*, window::PrimaryWindow, winit::WinitSettings};
 use bevy_egui::{EguiContexts, EguiPlugin};
 
 #[derive(Default, Resource)]
@@ -16,6 +16,7 @@ struct OriginalCameraTransform(Transform);
 
 fn main() {
     App::new()
+        .insert_resource(WinitSettings::desktop_app())
         .add_plugins(DefaultPlugins)
         .add_plugins(EguiPlugin)
         .init_resource::<OccupiedScreenSpace>()
@@ -26,6 +27,7 @@ fn main() {
 }
 
 fn ui_example_system(
+    mut is_last_selected: Local<bool>,
     mut contexts: EguiContexts,
     mut occupied_screen_space: ResMut<OccupiedScreenSpace>,
 ) {
@@ -35,6 +37,18 @@ fn ui_example_system(
         .resizable(true)
         .show(ctx, |ui| {
             ui.label("Left resizeable panel");
+            if ui
+                .add(egui::widgets::Button::new("A button").selected(!*is_last_selected))
+                .clicked()
+            {
+                *is_last_selected = false;
+            }
+            if ui
+                .add(egui::widgets::Button::new("Another button").selected(*is_last_selected))
+                .clicked()
+            {
+                *is_last_selected = true;
+            }
             ui.allocate_rect(ui.available_rect_before_wrap(), egui::Sense::hover());
         })
         .response
diff --git a/src/systems.rs b/src/systems.rs
index d03f64534..abd500e1e 100644
--- a/src/systems.rs
+++ b/src/systems.rs
@@ -14,11 +14,12 @@ use bevy::{
         ButtonState,
     },
     log,
-    prelude::{Entity, EventReader, Query, Resource, Time},
+    prelude::{Entity, EventReader, NonSend, Query, Resource, Time},
     time::Real,
     window::{CursorMoved, RequestRedraw},
+    winit::{EventLoopProxy, WakeUp},
 };
-use std::marker::PhantomData;
+use std::{marker::PhantomData, time::Duration};
 
 #[allow(missing_docs)]
 #[derive(SystemParam)]
@@ -482,6 +483,7 @@ pub fn process_output_system(
     mut egui_clipboard: bevy::ecs::system::ResMut<crate::EguiClipboard>,
     mut event: EventWriter<RequestRedraw>,
     #[cfg(windows)] mut last_cursor_icon: Local<bevy::utils::HashMap<Entity, egui::CursorIcon>>,
+    event_loop_proxy: Option<NonSend<EventLoopProxy<WakeUp>>>,
 ) {
     let mut should_request_redraw = false;
 
@@ -530,6 +532,21 @@ pub fn process_output_system(
         let needs_repaint = !context.render_output.is_empty();
         should_request_redraw |= ctx.has_requested_repaint() && needs_repaint;
 
+        // The resource doesn't exist in the headless mode.
+        if let Some(event_loop_proxy) = &event_loop_proxy {
+            // A zero duration indicates that it's an outstanding redraw request, which gives Egui an
+            // opportunity to settle the effects of interactions with widgets. Such repaint requests
+            // are processed not immediately but on a next frame. In this case, we need to indicate to
+            // winit, that it needs to wake up next frame as well even if there are no inputs.
+            //
+            // TLDR: this solves repaint corner cases of `WinitSettings::desktop_app()`.
+            if let Some(Duration::ZERO) =
+                ctx.viewport(|viewport| viewport.input.wants_repaint_after())
+            {
+                let _ = event_loop_proxy.send_event(WakeUp);
+            }
+        }
+
         #[cfg(feature = "open_url")]
         if let Some(egui::output::OpenUrl { url, new_tab }) = platform_output.open_url {
             let target = if new_tab {