Appearance of the block at the top of the window?

Question

css

How to implement the appearance of an absolutely positioned element at the top of the window. that is, to calculate in some way the coordinates

Answer 1

не пойму в чем проблема, абсолютное позиционирование уже подразумевает родителя в виде всего окна, можно указать напрямую значение top left и тд. если я понял о чем идет речь

Answer 2

If I understand everything, then through position: absolute
And when opening, write
Well, add an indent from the top of the screen.
Then the modal window will appear in the place where you called it, and when scrolling, it will remain in the same place.
https://jsfiddle.net/2x1qmg9q/