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BlockFakeInput is a sandbox setting in Sandboxie Ini. It specifies whether Sandboxie will allow sandboxed programs to manufacture fake keyboard input and send it to windows of applications running outside that sandbox.
. . . [DefaultBox] BlockFakeInput=n
Keyboard input is received by the active, highlighted window. This is true whether the keyboard input was manufactured by a program (fake input), or coming from the keyboard (real input).
By default, Sandboxie will allow a program running in a sandbox to manufacture fake input, provided the recipient window belongs to an application which is running in the same sandbox.
If the fake input will end up in a window outside that sandbox, Sandboxie will discard the input and issue message SBIE1304.
Specifying BlockFakeInput=n indicates that a sandboxed program should be allowed to manufacture fake keyboard input, regardless of the recipient of that input.
To experiemnt with this setting, you can run a sandboxed instance of osk.exe, the Windows on-screen keyboard.