FB24536235
On Mac Catalyst under macOS 27, a plain CADisplayLink asking for CAFrameRateRange(minimum: 60, maximum: 60, preferred: 60) gets serviced at 30 Hz for as long as a RealityView is on screen in the same window. The link does nothing per tick but count, so there's nothing of mine to blame it on. RealityKit's own statistics overlay reads 60.41 fps in the same frame.
Click a segmented control that removes the RealityView and the same link goes straight back to 60. Nothing else changes. That's the whole reproducer, and I've attached it to the radar.
It only happens while the display panel is in ProMotion mode. Set the built-in to a fixed 60 Hz and it's correct again. With an external 60 Hz display attached the roles swap: the built-in is fine and the external drops to somewhere between 18 and 30, and setting the built-in to 60 Hz fixes that one too without touching the external's own settings.
A raw MTKView presenting continuously at 60, at 120, and on a 120 Hz link presenting every second callback are all fine, so it isn't continuous presentation and it isn't the requested rate. It's RealityKit specifically. Worth knowing if you're testing: RealityView on Catalyst is an ARView underneath, so both paths give you the same answer.
This is VERY rough for anything that puts RealityKit next to a UI. In an editor that's the sidebar, the inspector, gizmos, drag handles, every display-link-driven or UIKit animation in the window running at half rate around a viewport that stays smooth.
Likely Related to FB24091347, which is the same defect seen as SwiftUI scroll judder. If you can reproduce either, please file a duplicate.
Attached two screenshots; first with promotion enabled, second with promotion off.
PLEASE fix this, it drives me crazy and there seems to be no workaround. On release day of macOS 27 our app will likely be blamed for it by users and my hands are tied.
Thank you!