know about overlfow of string in Floating field as Text field

i have requirement to know about the overflow of the floating field as depicted in snapshot account name is overflowed if name exceeds the length of floating field.

i have tried javaScript event.fieldFull but it returns undefined is there any alternative solution to find out overflow condition?

i can not use the length logic means count the number of character that floating field can hold so please suggest some other solution.

Thanks
Jitendra