targetOffset seems not works in the way i think

[luqingxuan]

i have the config bellow, $input is a input element, $picker is a div element, as popover layer

target: $input,
targetAttachment: 'bottom left',
 targetOffset: '10px 0',
element: $picker,
attachment: 'top left', 
constraints: [{
         to: 'window',
         attachment: 'together'
}]

u can see, $input & $picker has vertical offset 10px, when $picker show bellow $input, it works well,
but when scroll down browser, $picker flips, and show up $input, the 10px calculation seems wrong, the gap between $input & $picker seems -10px, not 10px,

maybe u can see this example, it uses tether for react-datepicker.
https://hacker0x01.github.io/react-datepicker/
datepicker plugin works well, because the author make js config:
targetOffset: '10px 0'

and also css:
.react-datepicker__tether-element-attached-bottom.react-datepicker__tether-element {
margin-top: -20px;
}

just multi 2....
so i think if he config targetOffset 20px, maybe he must writes margin-top: -40px,

btw: please forget my poor english.