Sure enough, on Theo's M2 MacBook, we indeed observed Zed dropping frames that weren't visible on our M1s, so we enabled the metal HUD on his copy of Zed to investigate. By default, presenting to a CAMetalLayer does not block drawing of the window by the OS, forcing the system to interpolate the window's contents from the previous frame by stretching them until the contents arrive on the next frame. Then I found out that if I turn off ProMotion for my MacBook's display and turn it back on again, macOS asks Zed for for more frames again — back to 120 FPS.