fill-paragraph causes screen jump as if it is calling

fill-paragraph (from fill.el) causes point to recenter alternately to center, top, and bottom of screen, as if recenter-top-bottom (from window.el) is being called.

I expect fill-paragraph to be idempotent: if it does nothing, it should leave the window alone (possibly marking it as dirty). In particular, it is hard to see exactly what it has done if the text moves substantially. This is new behavior since the upgrade from 14.10.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: emacs 46.1
ProcVersionSignature: Ubuntu 3.19.0-16.16-generic 3.19.3
Uname: Linux 3.19.0-16-generic i686
ApportVersion: 2.17.2-0ubuntu1
Architecture: i386
CurrentDesktop: i3
Date: Thu May 7 16:54:19 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2010-03-13 (1881 days ago)
InstallationMedia: Ubuntu-Netbook-Remix 9.10 "Karmic Koala" - Release i386 (20091028.4)
PackageArchitecture: all
SourcePackage: emacs-defaults
UpgradeStatus: Upgraded to vivid on 2015-05-06 (0 days ago)