Function render_pane_focus_ring 

pub fn render_pane_focus_ring<T: Draw>(
    target: &mut T,
    rect: Rect,
    ring: &PaneFocusRing,
)

Draw a focus ring around rect on any render target (Frame or Buffer).

Visually stable: it draws a deterministic border and never depends on animation or timing, so the active pane is unambiguous after any command.