I was today working on somebody else's Mac (OS X 10.9.2) and wanted to see Mail and Safari next to each other because I wanted to do something in Safari that was described in an email in Mail. However, it was not possible to see both applications at the same time. Whenever I switched via the Dock from one application to the other the first application would disappear as if hidden. The person who owns the computer said to me that this would happen frequently but not always and she would not know what triggers it. We could solve this by quitting and restarting both Safari and Mail - both applications could then be seen again simultaneously. Does someone know how this strange behavior gets triggered and how it can be avoided from the very beginning?