double click in Mail no longer works
I'm running Snow Leopard on a mid-2011 iMac and have Mail version 4.6. Everything has worked for all those years, but a few days ago I lost the ability to view messages in a separate window by double-clicking.
I saw some old threads about this topic, but it looked complicated and a bit scary (repairing permissions, etc) so I prefer to ask here if there might be a simpler answer...
Thanks in advance
iMac, Mac OS X (10.6.8)