The past couple of days I have noticed an issue with the wp editor and its containment within the editor box when scrolling.
I find that the 'right' admin sidebar scrolls in conjunction with the editor window as the document grows in length and I find that the option/ability to size the editor window has been missing for the same few days.
I have tested with all plugins deactivated and using the 2014, 2013 and 2010 default themes with no differences found.
this problem first appeared to me on July 7 or 8 of 2014 as I recall.
I do have several test sites I can make available to review the issue if unable to duplicate yourself...