Product: Synergy (all versions)
Operating system: Windows
Many full screen applications, particularly games (and RDPs), automatically minimize when they're no longer the active (foreground) application on Microsoft Windows.
The Synergy server normally becomes the foreground when you switch to another screen in order to more reliably capture all user input causing those full screen applications to minimize.
To prevent Synergy from using the foreground window:
In Synergy 1: Configure Server --> Advanced Server Options and check Don't take foreground window on Windows servers.
Synergy 3: Miscellanous > Uncheck "Take foreground window" option
If you turn this on then be aware that Synergy may not function correctly when certain programs, particularly the command prompt, are the foreground when you switch to other screens. Simply make a different program the foreground before switching to work around that.