There is a page in which the colors of the blocks alternate. In each block there should be a menu opening button: in the black block there is a white button, in white - black.
How to implement such an effect that when scrolling the page the button changes color depending on the block. As if one button goes beyond the border of the parent unit, and the other leaves. Tried to do through overflow: hidden , but it does not work on the button with fixed positioning.
An example of what is needed.
