Cribbed from bevy flycam plugin, works much better

src/camera.rs

@@ -1,3 +1,5 @@
+use bevy::input::mouse::MouseButtonInput;
+
 use crate::prelude::*;
 
 /// Menu Plugin; empty struct for Plugin impl
@@ -13,11 +15,13 @@ impl Plugin for CameraPlugin {
 pub(crate) struct FlyCamera;
 
 /// Fly camera system for moving around like a drone
+/// TODO: Only if key is pressed!
 fn editor_fly_camera(
 	mut cameras: Query<(&Camera, &mut Transform), With<FlyCamera>>,
 	windows: Query<&Window>,
 	primary_window: Query<Entity, With<PrimaryWindow>>,
 	keys: Res<ButtonInput<KeyCode>>,
+	mouse: Res<ButtonInput<MouseButton>>,
 	mut cursor_events: EventReader<CursorMoved>,
 	time: Res<Time>,
 ) {
@@ -35,8 +39,10 @@ fn editor_fly_camera(
 				},
 				_ => None,
 			};
+			let window = windows.get(target_window.unwrap()).unwrap();
+
 			// If the target window is focused
-			windows.get(target_window.unwrap()).unwrap().focused.then(|| {
+			window.focused.then(|| {
 				let move_speed = 4.0;
 				let mut delta = Vec3::ZERO;
 				if keys.pressed(KeyCode::KeyW) {
@@ -60,26 +66,24 @@ fn editor_fly_camera(
 				t.translation += delta;
 			});
 
+			if mouse.pressed(MouseButton::Middle) {
 				cursor_events.read().filter_map(|CursorMoved { delta, window, .. }| {
 					(*window == target_window.unwrap()).then_some(delta)
 				}).for_each(|delta| {
-				// If there was actual movement, unpack those values
 					if let Some(Vec2 { x, y }) = delta {
-					if x > y {
-						// Find the horizontal angle of movement
-						let width = 1280.0;
-						let ratio_width = x / width;
-						let angle_width = ratio_width * std::f32::consts::PI;
-						t.rotate_local_y(-angle_width);
-					} else {
-						// Find the vertical angle of movement
-						let height = 720.0;
-						let ratio_height = y / height;
-						let angle_height = ratio_height * std::f32::consts::PI;
-						t.rotate_local_x(-angle_height);
-					}
+						// Cribbing from bevy_flycam
+						// Link: https://github.com/sburris0/bevy_flycam/blob/baffe50e0961ad1491d467fa6ab5551f9f21db8f/src/lib.rs#L145-L151
+						let (mut yaw, mut pitch, _) = t.rotation.to_euler(EulerRot::YXZ);
+						let window_scale = window.height().min(window.width());
+						let sensitivity = 0.00012;
+						pitch -= (sensitivity * y * window_scale).to_radians();
+						yaw -= (sensitivity * x * window_scale).to_radians();
+						t.rotation = Quat::from_axis_angle(Vec3::Y, yaw) * Quat::from_axis_angle(Vec3::X, pitch);
 					}
 				});
+			} else {
+				cursor_events.clear();
+			}
 		});
 	});
 }

src/editor.rs

@@ -104,8 +104,8 @@ fn origin_directions(
 fn world_plane(
 	mut gizmos: Gizmos
 ) {
-	(-10..10).into_iter().for_each(|x| {
-		(-10..10).into_iter().for_each(|z| {
+	(-10..=10).into_iter().for_each(|x| {
+		(-10..=10).into_iter().for_each(|z| {
 			{
 				let start = Vec3::new(x as f32, 0.0, -10.0);
 				let end = Vec3::new(x as f32, 0.0, 10.0);