It's not only buttons, sometimes the whole UI moves. Many mobile websites use sticky headers and footers that stick or unstick, appear or hide according to the position of the users in the document, how long since they touched the screen, scrolled, etc. I either switch to reader mode or savagely use uBO's element picker to select and permanently hide those elements. At the end of the treatment either the header displays only when I scroll to the top and the footer when I scroll to the end or they never display. I really don't care because what I want to read is the content in between.