currently the 'showing_desktop_changed' signal is emitted when the root setting is explicitely toggled using NET_SHOWING_DESKTOP. However for the purposes of the showdesktop panel plugin (not yet in svn) which unlike the one in 4.2 uses a toggle button, it would be nice to emit this signal when windows are deiconified too, so the button is updated to reflect that state is 'unshow'. Reproducible: Always Steps to Reproduce: 1. 2. 3.
Actually, that was on purpose (kind of different approach for showing desktop, more like a flag that hides are restore all windows ven if new windows show in between). Anyway, the suggested behaviour is now implement in SVN trunk.