FB24091347
On macOS 27.0 beta (26A5388g), MacBook Pro M4 Pro, the built-in ProMotion display never settles on a stable refresh cadence. Scrolling in SwiftUI judders constantly. The same app binary was smooth on macOS 26, and is smooth on a 120 Hz ProMotion iPad.
I captured two 60-second Instruments traces — same app, same scene, same scrolling, no external display — changing only the display's refresh-rate setting. On ProMotion the vsync interval standard deviation is 4.093 ms across six different cadences, mostly flip-flopping between 120 Hz and 60 Hz. Forced to a fixed 60 Hz it drops to 0.391 ms with a single cadence. The app presented an identical 59 fps median in both runs — frame production is perfectly steady, the display just holds each frame for an unpredictable length of time.
That's what makes this nasty: it's invisible to every frame-rate metric, so it looks like the app got slow when nothing about the app changed. I spent most of a day profiling my own code before realising the app was never the problem.
Workaround: force the built-in display to 60 Hz.
Worth noting, because it complicates the picture: attaching a 60 Hz Studio Display makes the built-in smooth, but the Studio itself then judders — despite its own vsync cadence measuring perfectly stable. So refresh rate alone isn't the whole story, and there may be a second mechanism. The clean, reproducible, single-variable result is the ProMotion vs forced-60 Hz comparison on the built-in panel.
If you can reproduce this on an M-series MacBook Pro on 27 beta, please file a duplicate referencing FB24091347.