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How to change max height of container?
Default max height of BP Better Messages container for desktops is set to 500px.
There is bp_better_messages_max_height filter which you can use to change that value, this is example of how to use it:
add_filter('bp_better_messages_max_height', function($max_height){ return 600; // New max height in PX });
Perfect place to put this snippet is child theme functions.php file.