Self Styled modal in the template of react-pure-modal

[Mortuie]

I was wondering if anyone else had come up with a similar issue, when styling/using a modal. The highlighted area in blue signifies where the mozilla firefox url bar goes. This is on an android mobile phone using Mozilla Firefox.

When i then scroll up on the modal itself the url bar appears. The styling I have used is inspired by the styling for the react pure modal components.