Find an element with a transition, and click on the little easing icon in the transition style. Drag the handles around. Note that you end up selecting text in the style sidebar behind the popup. The popup needs to stop event propagation. <rdar://problem/59208828>
Created attachment 389913 [details] Patch
Comment on attachment 389913 [details] Patch Clearing flags on attachment: 389913 Committed r255888: <https://trac.webkit.org/changeset/255888>
All reviewed patches have been landed. Closing bug.