Bug 825499 added caretPositionFromPoint to nsDocument. It's possible that nsLayoutUtils::GetFrameForPoint (used in the implementation of this function) can return a frame from a child document, but GetOffsetTo (also used) does not work cross-document. We need to determine if supporting cross-document caret positions is what we actually want to do, given that it might have the potential to retrieve information from a sub document in a non-secure way. If we decide to proceed with this, we need to change GetOffsetTo to call GetOffsetToCrossDoc instead.
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