There's a problem with that too: it would often force the owner of the phone to input the code when outside a secure environment, so a simple video surveillance could obtain the code.

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cherryteastain 10 minutes ago | parent [-]

GrapheneOS has a feature to scramble the locations of the numbers on the passcode unlock screen to prevent this exact problem

	▲	ragall 3 minutes ago \| parent [-]
		That can be helpful only if surveillance cannot film the screen directly, and surveillance relies on the position of the fingers. Given modern 4k cameras it's unlikely the latter can be done but not the former.