Minimizing the window, or switching virtual desktops leads to UnmapNotify (on X11) which hides the toplevel window, which calls this code in qdockwidget.cpp:

case QEvent::Hide:
if (layout != 0)
layout->keepSize(this);
d->toggleViewAction->setChecked(false);
emit visibilityChanged(false);
break;

If the app connects visibilityChanged(false) to a slot which toggles a QAction, then the dockwidget will not be shown again when switching back to the virtual desktop where the app is.

I think there's a missing check for event->spontaneous() in that code?