That spark will go to ground because the case is grounded.
And the inputs are protected by their input capacitors, which can cope easily with static voltage induced power. It may be several thousand volts, but there is very little energy in static voltage.
A direct lightning strike is a completely different matter. There are no electronics capable of surviving that.
And very high impedance inputs (Hi-Z, like for instrument inputs) are more vulnerable too. But you'll jump of your chair before these get damaged.
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