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Automatic taskbar masking is not supported on Windows 7 Tablet PCs where only pen or touch input is used without a keyboard or mouse.
If you enable auto fade, it can cause the taskbar to minimize beyond the touchscreen area. This becomes even clearer when you configure the calibration option in Control Panel > Tablet PC Settings.
Calibration can further reduce the touchscreen surface at the edge of the screen to exclude the area needed to activate and restore the hidden taskbar.
To disable automatic hiding of the taskbar, open the Control Panel and select Taskbar and Start menu. On the Taskbar tab, turn off the Hide Taskbar Automatically option.
The default setting for this option in Windows 7 is disabled, but if you have it enabled, you can disable it, KB2495079 recommends !