wezterm/docs/config/lua/mux-window/active_pane.md

window:active_pane()

{{since('nightly')}}

A convenience accessor for returning the active pane in the active tab of the window.

In earlier versions of wezterm, you could obtain this via:

function active_tab(window)
  for _, item in ipairs(window:tabs_with_info()) do
    if item.is_active then
      return item.tab
    end
  end
end

function active_pane(tab)
  for _, item in ipairs(tab:panes_with_info()) do
    if item.is_active then
      return item.pane
    end
  end
end

See also gui_window:active_pane(), which is similar to this method, but which can return overlay panes that are not visible to the mux layer of the API.