Function acquire_focus_tab_index 

pub fn acquire_focus_tab_index(
    acquire_focus: On<'_, '_, AcquireFocus>,
    focusable: Query<'_, '_, (), (With<TabIndex>, Without<Window>)>,
    focus: ResMut<'_, InputFocus>,
)

Observer which focuses the target of an AcquireFocus request when it has a TabIndex.

This is the tab-navigation-specific half of focus acquisition: only entities that opt into tab navigation via TabIndex are treated as focus targets here. When the target is focusable, this stops the request (so it never reaches the window) and focuses it; otherwise the request keeps bubbling and is eventually handled by the generalized on_window_acquire_focus_clear observer at the window.

The Without<Window> bound is a defensive guard — a window would never carry a TabIndex in practice, but excluding it keeps this observer’s responsibility (focus a focusable child) cleanly separate from the window-clearing fallback in on_window_acquire_focus_clear.

The focus.get() guard avoids spurious mutations so change detection only fires on real changes.