fixes facebookarchive#304, kudos to @andpor (facebookarchive#2197)

Summary:
Should fix facebookarchive#304.
Thanks andpor.

**Summary**
based on Andrzej's facebookarchive#1223, issue has been there for an incredible 3.5 years. Let's fix this.
Pull Request resolved: facebookarchive#2197

Reviewed By: elboman

Differential Revision: D18314441

Pulled By: mrkev

fbshipit-source-id: 99284101bab838c574341b44b3cfcd935f1dc0e2

src/component/contents/DraftEditorBlock.react.js

@@ -126,7 +126,9 @@ class DraftEditorBlock extends React.Component<Props> {
         'blockNode is not an HTMLElement',
       );
       const blockBottom = blockNode.offsetHeight + blockNode.offsetTop;
-      const scrollBottom = scrollParent.offsetHeight + scrollPosition.y;
+      const pOffset = scrollParent.offsetTop + scrollParent.offsetHeight;
+      const scrollBottom = pOffset + scrollPosition.y;
+
       scrollDelta = blockBottom - scrollBottom;
       if (scrollDelta > 0) {
         Scroll.setTop(