(Bug 41231) Fix position and size of container

- Offset the position of the PostEdit message by a mixed
  measurement rather than relative to the browser window
  height.

- Set container height to 0.

Fixes proposed by TMg:
https://bugzilla.wikimedia.org/show_bug.cgi?id=41231#c20

Patch Set #2:
Convert 10px -> 0.6em for consistency with em sizing,
per TMg's suggestion in Bugzilla.

Change-Id: Idf9b2020e30eef21d4e3b809f4eec634f786c722
Merge "(Bug 41231) Fix position and size of container"