You might have turned on
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>When Screen Curtain is on, your device and VoiceOver navigation stay active, even though your display is turned off. For example, if you want to check your bank account in a public place, Screen Curtain turns your screen off so that your information isn't visible.
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> To turn Screen Curtain on or off by gesture:
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> * With three fingers, quickly triple-tap the screen. If the Zoom feature is in use with VoiceOver, tap the screen four times with three fingers.
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> If you use a bluetooth keyboard with your device, press Control-Option-Shift-S to turn Screen Curtain on or off.