M3 MacBook Air (screen open) + two external screens simultaneously

[MacPeasant123]

I discovered recently that with my 15 inch M3 MacBook Air (which Apple says only supports two external monitors when the MBA's lid is closed), I can use it with the lid open along with a 15 inch portable monitor and sidecar via USB-C cable with a 2020 iPad Pro (12.9"), for a total of three screens simultaneously.

I'm pleasantly surprised this worked. Maybe it's because my portable monitor and iPad Pro aren't large screens, so it's not taxing whatever external output engine is being used?

 

[mr_roboto]

I discovered recently that with my 15 inch M3 MacBook Air (which Apple says only supports two external monitors when the MBA's lid is closed), I can use it with the lid open along with a 15 inch portable monitor and sidecar via USB-C cable with a 2020 iPad Pro (12.9"), for a total of three screens simultaneously.

I'm pleasantly surprised this worked. Maybe it's because my portable monitor and iPad Pro aren't large screens, so it's not taxing whatever external output engine is being used?

Correct - iPad sidecar mode doesn't use any of the MBA's display engines. The Mac draws into a frame buffer that is not connected to a display engine, compresses it using one of its video codec accelerator blocks, and sends the data to the iPad over USB or WiFi.

Contrast this with a "real" display. With one of these, the Mac still draws into a frame buffer, but this time one of its display engines is configured to scan that buffer at the display refresh rate and format the refresh data as a DisplayPort packet stream, which is sent out one of the Type C ports (usually in DisplayPort Alt mode, sometimes in Thunderbolt mode).

The display engines responsible for creating DisplayPort signal are the piece of hardware your MBA has only two of. When the MBA's lid is open, one engine is allocated to the MBA's internal display (which itself is an embedded DisplayPort device). If you close the lid, the M3 chip can be reprogrammed to redirect the internal display's engine to drive an external monitor.

Hope that makes sense.