Easing Fits Subject

Pick the curve from what is changing, in this order: elements entering or exiting the screen → ease-out (and, on this reading, ease-in is also permitted for exits, on the argument that a departing element may accelerate away); an element morphing or moving between two on-screen states → ease-in-out; hover and other pointer feedback → ease; anything constant or continuous, like a marquee or a spinner → linear.

Then match the magnitude to the distance travelled: a 4px hover nudge and a full-screen sheet should not share a curve. Note the field disagrees about exits — "Never ease-in on UI" (animations-emil-no-ease-in) rejects ease-in outright, on the grounds that it stalls the first ~100ms, which is exactly the window in which the user judges speed. Both entries are kept deliberately: read them together and decide, rather than inheriting one by default.