Modal

Centered modal with inner scrollbar.
Try some modals:

Recommended: Choose a design pattern and stick to it. For example, always include either the top-right close button, or the bottom-left Cancel button, or both. But don't alternate between patterns.

Design References
Bootstrap Modal
Material Dialog
Medium: A small collection of ...
Dependencies
Bootstrap and jQuery (or you could re-use the css in your own modal script).
Browsers
Tested in Chrome, Firefox, Edge, IE9+.

See the Pen Bootstrap Modal (centered) by btn.ninja (@btn-ninja) on CodePen.