title: Flutter Google Maps, how to detect map area change by user, not by controller
body: <p>I am using Google Maps to display markers on the map and other widgets to describe each marker on the map. I have successfully implemented when the user tab on a widget, the map area will change to make the matching marker to be the center of the map.
  I also have the 'Search this area' button to search the map area.
  The hard part is to control how to hide/display the 'Search this area' button.
  So far, I have implemented it to display when the map area is changed. However, it causes the button to appear every time the map area changed by the user tab on a widget.</p>
  <p>I would like to only detect the map area change by the user manually such as pinch zoom in / out, drag etc.</p>