The Avatar component was initializing maskUrl as '' and loading it in a
useEffect. Since hasCustomShape was true immediately, rounded-full was
removed on the first render, but the mask wasn't applied until after the
effect fired — causing a visible square flash for one frame.
getAvatarMaskUrl is already synchronous (renders emoji to canvas, caches
the data-URL), so compute it inline during render instead of deferring
to an effect. The mask is now applied on the very first paint.
Alex Gleason
02231ea1f9