Function write_mouse_wheel_messages_system 

pub fn write_mouse_wheel_messages_system(
    modifier_keys_state: Res<'_, ModifierKeysState>,
    mouse_wheel_reader: EguiContextMessageReader<'_, '_, MouseWheel>,
    egui_input_message_writer: MessageWriter<'_, EguiInputEvent>,
    egui_contexts: Query<'_, '_, &EguiContextSettings, With<EguiContext>>,
)

Reads MouseWheel messages and wraps them into EguiInputEvent, can redirect messages to HoveredNonWindowEguiContext.