top panel and launcher are unresponsive

Unity has recently (~2014-07-15) started exhibiting two odd behaviors that are likely related:
1) The launcher, which is set to autohide, will appear and disappear in <1s (even though I hover over the launcher). It is unresponsive to clicks.
2) All applets in the top panel are unresponsive to clicks.

1 & 2 always occur together at some point during a session. That is, these components work correctly for hours (days?), then become unresponsive. I have not yet identified the trigger event; screenlocking or suspend/resume *may* be related, but not reliably so (i.e., I can't reproduce it this way).

Restarting unity-panel-service (killall unity-panel-service) doesn't help. The problem is temporarily fixed by logging out/in or rebooting, but the issue recurs.

The timing of the first occurrence (in July) is probably consistent with my first use of unity-tweak-tool to set focus follows mouse, although this hypothesis doesn't explain why the components initially work and then become unresponsive. Otherwise, my config is pretty stock.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity 7.2.2+14.04.20140714-0ubuntu1.1
ProcVersionSignature: Ubuntu 3.13.0-32.57-generic 3.13.11.4
Uname: Linux 3.13.0-32-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.2
Architecture: amd64
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,imgpng,mousepoll,snap,resize,move,grid,unitymtgrabhandles,place,regex,gnomecompat,vpswitch,animation,expo,workarounds,ezoom,wall,session,fade,scale,unityshell]
CurrentDesktop: Unity
Date: Sun Aug 3 12:17:34 2014
InstallationDate: Installed on 2012-10-09 (662 days ago)
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
SourcePackage: unity
UpgradeStatus: Upgraded to trusty on 2014-04-21 (103 days ago)