if (new_messages_height <= scroll_limit) {                                                       
            // This is the happy path where we can just scroll                                           
            // automatically, and the user will see the new message.                                     
            scroll_amount = new_messages_height;                                                         
            need_user_to_scroll = false;                                                                 
        } else {                                                                                         
            // Sometimes we don't want to scroll the entire height of                                    
            // the message, but our callers can give appropriate                                         
            // warnings if the message is gonna be offscreen.                                            
            // (Even if we are somewhat constrained here, the message may                                
            // still end up being visible, so we do some arithmetic.)                                    
            scroll_amount = scroll_limit;                                                                
            const offset = $message_viewport.offset_from_bottom($last_visible);                          
                                                                                                         
            // For determining whether we need to show the user a "you                                   
            // need to scroll down" notification, the obvious check                                      
            // would be `offset > scroll_amount`, and that is indeed                                     
            // correct with a 1-line message in the compose box.                                         
            // However, the compose box is open with the content of                                      
            // the message just sent when this code runs, and                                            
            // `offset_from_bottom` if an offset from the top of the                                     
            // compose box, which is about to be reset to empty.  So                                     
            // to compute the offset at the time the user might see                                      
            // this notification, we need to adjust by the amount that                                   
            // the current compose is bigger than the empty, open                                        
            // compose box.                                                                              
            const compose_textarea_default_height = 42;                                                  
            const compose_textarea_current_height = $("#compose-textarea").height();                     
            const expected_change =                                                                      
                compose_textarea_current_height - compose_textarea_default_height;                       
            const expected_offset = offset - expected_change;                                            
            need_user_to_scroll = expected_offset > scroll_amount;                                       
        }                                                                                                
                                                                                                         
        // Ok, we are finally ready to actually scroll.                                                  
        if (scroll_amount > 0) {                                                                         
            $message_viewport.system_initiated_animate_scroll(scroll_amount);                            
        }                                                                                                

export function system_initiated_animate_scroll(scroll_amount) {                                         
    message_scroll.suppress_selection_update_on_next_scroll();                                           
    const viewport_offset = scrollTop();                                                                 
    in_stoppable_autoscroll = true;                                                                      
    $message_pane.animate({                                                                              
        scrollTop: viewport_offset + scroll_amount,                                                      
        always() {                                                                                       
            in_stoppable_autoscroll = false;                                                             
        },                                                                                               
    });                                                                                                  
}