Full-screen page transitions, mostly horizontal. Tap a card to open a page with that transition; go back with the Back button, the browser/phone back, Esc, or by swiping right on the page (the transition scrubs with your finger).
Jank test blocks the main thread for ~360 ms during each transition: GPU-only transitions should keep moving smoothly, anything that needs the main thread will stutter. The FPS pill counts main-thread frames, so it drops during the jank test even when the motion itself stays smooth.