GPT-samples-collapse-empty-ad-slots Ad Manager docs
By default, ad slots which are not filled are left visible, which may result in blank space on your page. If you know ahead of time that one or more ad slots on your page are unlikely to get filled, you can instruct the Google Publisher Tag (GPT) library to collapse them. The optimal configuration of this feature will depend on how often you expect ad slots to be filled: If slots will be filled most of the time, use collapseDiv: 'ONNOFILL'. In this configuration, ad slots are expanded by default and collapse only if they cannot be filled. If slots will not be filled most of the time, use colla
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