From 4122ff28d44a2c965ece074f67d9e212d44118db Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fran=C3=A7ois?= <mockersf@gmail.com>
Date: Sat, 21 Jan 2023 00:01:28 +0000
Subject: [PATCH] break feedback loop when moving cursor (#7298)

# Objective

- Fixes #7294

## Solution

- Do not trigger change detection when setting the cursor position from winit

When moving the cursor continuously, Winit sends events:
- CursorMoved(0)
- CursorMoved(1)
- => start of Bevy schedule execution
- CursorMoved(2)
- CursorMoved(3)
- <= End of Bevy schedule execution

if Bevy schedule runs after the event 1, events 2 and 3 would happen during the execution but would be read only on the next system run. During the execution, the system would detect a change on cursor position, and send back an order to winit to move it back to 1, so event 2 and 3 would be ignored. By bypassing change detection when setting the cursor from winit event, it doesn't trigger sending back that change to winit out of order.
---
 crates/bevy_winit/src/lib.rs                         | 12 ++++++++++--
 .../tools/scene_viewer/camera_controller_plugin.rs   |  3 ++-
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/crates/bevy_winit/src/lib.rs b/crates/bevy_winit/src/lib.rs
index ff4eda67c5260..84303caaa1c99 100644
--- a/crates/bevy_winit/src/lib.rs
+++ b/crates/bevy_winit/src/lib.rs
@@ -396,7 +396,11 @@ pub fn winit_runner(mut app: App) {
                             window.resolution.physical_height() as f64 - position.y,
                         );
 
-                        window.set_physical_cursor_position(Some(physical_position));
+                        // bypassing change detection to not trigger feedback loop with system `changed_window`
+                        // this system change the cursor position in winit
+                        window
+                            .bypass_change_detection()
+                            .set_physical_cursor_position(Some(physical_position));
 
                         cursor_events.cursor_moved.send(CursorMoved {
                             window: window_entity,
@@ -412,7 +416,11 @@ pub fn winit_runner(mut app: App) {
                     WindowEvent::CursorLeft { .. } => {
                         // Component
                         if let Ok((mut window, _)) = window_query.get_mut(window_entity) {
-                            window.set_physical_cursor_position(None);
+                            // bypassing change detection to not trigger feedback loop with system `changed_window`
+                            // this system change the cursor position in winit
+                            window
+                                .bypass_change_detection()
+                                .set_physical_cursor_position(None);
                         }
 
                         cursor_events.cursor_left.send(CursorLeft {
diff --git a/examples/tools/scene_viewer/camera_controller_plugin.rs b/examples/tools/scene_viewer/camera_controller_plugin.rs
index 701947bb775e8..2c2dcac4522bf 100644
--- a/examples/tools/scene_viewer/camera_controller_plugin.rs
+++ b/examples/tools/scene_viewer/camera_controller_plugin.rs
@@ -177,7 +177,8 @@ fn camera_controller(
             for mouse_event in mouse_events.iter() {
                 mouse_delta += mouse_event.delta;
             }
-        } else {
+        }
+        if mouse_button_input.just_released(options.mouse_key_enable_mouse) {
             for mut window in &mut windows {
                 window.cursor.grab_mode = CursorGrabMode::None;
                 window.cursor.visible = true;