wrong image size with specific element, which includes margin or padding.

[ssut]

I am trying to take a screenshot with a selector for capture the specific element:

irb(main):016:0> f = Screencap::Fetcher.new('https://ubit.info/ssut')
irb(main):027:0> f.fetch(output: 'ssut.png',div: '#music_list')
url=https%3A%2F%2Fubit.info%2Fssut
output=ssut.png
div=#music_list
=> #<File:ssut.png>

then, the result above is:

btw, the expected result is following:

and the div area is as follows: