Function write_egui_input_system 

pub fn write_egui_input_system(
    focused_non_window_egui_context: Option<Res<'_, FocusedNonWindowEguiContext>>,
    window_to_egui_context_map: Res<'_, WindowToEguiContextMap>,
    modifier_keys_state: Res<'_, ModifierKeysState>,
    egui_input_reader: MessageReader<'_, '_, EguiInputEvent>,
    egui_file_dnd_message_reader: MessageReader<'_, '_, EguiFileDragAndDropMessage>,
    egui_contexts: Query<'_, '_, (Entity, &mut EguiInput)>,
    windows: Query<'_, '_, &Window>,
    time: Res<'_, Time<Real>>,
)

Reads both EguiFileDragAndDropMessage and EguiInputEvent messages and feeds them to Egui.