something continually steals focus, preventing interaction with any window

Sometimes after unlocking the screensaver, Gnome is unusable because no window can be focused.

Symptoms:
* When a window is clicked, its decoration flashes to focused and then immediately changes back to unfocused.
* The mouse cursor correctly changes when positioned over e.g. an input field, but no clicks get through to the application, nor do any keystrokes.
* Global keyboard shortcuts like Meta and Ctrl+Alt+⇩ sporadically work a few times, but mostly have no effect.
* The Gnome taskbar at the top of the screen accepts clicks and seems to work fine, but keyboard actions are also blocked.

Killing all applications (which requires logging in via SSH) doesn't resolve the problem. The entire Gnome session must be killed, then it's fine the next time until the problem recurs.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: gnome-shell 3.24.2-0ubuntu0.1
Uname: Linux 4.11.0-041100-generic x86_64
ApportVersion: 2.20.4-0ubuntu4.4
Architecture: amd64
Date: Tue Jul 11 08:03:13 2017
DisplayManager: gdm3
InstallationDate: Installed on 2016-12-02 (219 days ago)
InstallationMedia: Ubuntu-GNOME 16.10 "Yakkety Yak" - Release amd64 (20161012.1)
ProcEnviron:
 LANG=en_AU.UTF-8
 SHELL=/bin/bash
 TERM=screen
 LANGUAGE=en_AU:en
 PATH=(custom, no user)
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)