Inabox Scroll Position API

[zhouyx]

For inabox to functional properly, it requires scroll positions from host.

Position Data

{
  viewportRect: viewport rect relative to host doc,
  positionRect: inabox iframe rect relative to viewport,
}

Two Approaches to get position

• Host doc notifies inabox iframe its new position on window 'scroll'/'resize'.
• Inabox requests its position from host.

Privacy Concern

• Sending viewport scroll info to cross origin iframe.
• Sending position info to cross origin iframe when its not in viewport.

[zhouyx]

close with #10987 #11017