How to stop mobile safari from setting fixed positions to absolute on input focus?

Safari has supported position: fixed since at least version 9.2, but if you're seeing difficult issues, you can fully create the fixed position effect by making the document element and body full screen and then using absolute positioning. Scrolling then occurs in some main container element rather than the body. Your "fixed" elements can exist anywhere in the markup using this method.

jsfiddle here

html,
body,
.mainContainer {
  height: 100%;
  width: 100%;
  overflow: hidden;
  margin: 0;
}

.mainContainer {
  overflow: auto;
}

.fixed {
  position: absolute;
  bottom: 20px;
  left: 20px;
}