How Mac Duo turns your lid into glass

macOS exposes the angle of the lid on Macs that have the hardware for it. Mac Duo reads that value directly, 120 times a second while the lid is moving, and treats it as the position of the fold. Finding that sensor is its own problem: how Mac Duo reads your MacBook's lid angle covers the search and the checks it applies before trusting one.

The fold is measured from wherever your lid is resting rather than from a fixed angle, so it behaves the same whether you work at 100° or 130°. You can set that resting angle yourself, and Mac Duo remembers it between launches.

The glass is made out of your real screen. Mac Duo captures the display and renders the pane over it with Metal, which is why windows, wallpaper and menu bar all lean together as one surface instead of sliding as separate layers.

This is the reason the app needs Screen Recording permission. Those frames stay in memory, nothing is written to disk, and nothing is sent anywhere — the app has no analytics and no account system.

The pane lifts off the display as it tilts. The gap is nothing at the hinge and grows toward the top, so the bottom of your screen stays almost untouched while the top frosts over. There is grain in the material and no highlights, because etched glass scatters light rather than reflecting it.

Frost, perspective, edge softness and corner radius are all adjustable, and the corner rounding is there so the pane can match the shape of your own display.

Screen capture stops the moment the lid settles, and the fold drops to a stationary frame rate you choose: 15, 30, 60, 90 or 120. Fifteen is the default and the lowest, and a lower stationary rate does less work while the lid is sitting still. Moving the lid brings it straight back to full rate.

There is also a minimum-movement setting, so small adjustments while you work do not have to trigger the fold at all.

macOS 14.0 Sonoma or later, Screen Recording permission, and a MacBook with a continuous lid-angle sensor. That last one is hardware, so it is the only requirement you cannot change: what the lid-angle sensor is and why the fold needs it, and which MacBooks have one.

If you came here from the iPhone side of it, the iPhone Duo fold on a MacBook covers what is the same and what is not.