Displaying a widget in sidebar only when no other widgets rendered?

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I not 100% clear what your question is so hopefully this can point you in the right direction for conditional widgets. I will give you 3 options.

1. is_active_widget checks if widgets are active based in the widget ID.

2. Using is_active_sidebar and creating more sidebars usually gives you better control over widget output.

3. Use a plugin, there are several plugins that have post or conditional logic built into them.

4. Try to use the widget_display_callback filter for example:

add_filter('widget_display_callback','your_custom_function',10,3);