quick_info_title

specifies the title of the pop-up edit form

start	Date	the date when a task is scheduled to begin
end	Date	the date when a task is scheduled to be completed
task	Task	the task object

gantt.templates.quick_info_title = function(start, end, task){ 
       return ev.text.substr(0,50); 
};

Details

This template is defined in the Quick Info extension, so you need to activate the quick_info plugin.