Comment 48 for bug 1181666

In my case the bug was on gnome-shell extension "workspaces-to-dock"(https://github.com/passingthru67/workspaces-to-dock). I found this log entry:

Apr 24 10:33:35 hostname gnome-shell[2736]: JS ERROR: TypeError: this._porthole is undefined#012myThumbnailsBox<._getPreferredWidth@/home<email address hidden>/myWorkspaceThumbnail.js:1065:17#012wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22#012ThumbnailsSlider<._getPreferredWidth@/home<email address hidden>/dockedWorkspaces.js:194:37#012wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22#012_init/<@resource:///org/gnome/shell/ui/layout.js:209:17#012_loadBackground/signalId<@resource:///org/gnome/shell/ui/layout.js:593:13#012_emit@resource:///org/gnome/gjs/modules/signals.js:128:27#012_init/id<@resource:///org/gnome/shell/ui/background.js:523:17

After remove /home<email address hidden> and restart session the behavior described here disappears.