Function bevy_ui_navigation::systems::generic_default_mouse_input

pub fn generic_default_mouse_input<T: ScreenSize + Component, M: Component>(
    input_mapping: Res<'_, InputMapping>, 
    windows: Res<'_, Windows>, 
    mouse: Res<'_, Input<MouseButton>>, 
    touch: Res<'_, Touches>, 
    focusables: NodePosQuery<'_, '_, T, M>, 
    focused: Query<'_, '_, Entity, With<Focused>>, 
    nav_cmds: EventWriter<'_, '_, NavRequest>, 
    last_pos: Local<'_, Vec2>
)

A generic system to send mouse control events to the focus system

T must be a component assigned to Focusable elements that implements the ScreenSize trait. M is simply a marker component for your camera entity, if no entities has the M component, we assume the screen space is not offset from the GlobalTransform of focusable elements.

Which button to press to cause an action event is specified in the InputMapping resource.

You may however need to customize the behavior of this system (typically when integrating in the game) in this case, you should write your own system that sends NavRequest events. You may use ui_focusable_at to tell which focusable is currently being hovered.