A tooltip that no longer eats your click
Hover a button, then press fast, before its label even appears. The click lands. The label no longer slips its own little render between your press and your release.
You sweep the pointer onto a commit's delete button and click before the label has had its 150 milliseconds. The confirm appears on that first press. It did not always. In WebKit the click simply vanished, and you clicked again, and only then did the commit go.
Credit goes to jubianchi, who found the seam between hovering and pressing. Tooltips here stay off the page until someone points at them, and the first hover mounts the real trigger, which means a brand new button appears under your cursor. React used to do that mounting after the hover event finished, so your press hit the old button and your release hit the new one, and the browser counted no click at all. jubianchi now forces the mount on the spot, inside the hover itself, so the button you press is the button you release.
There is one catch. It arms for a mouse and for a keyboard tab only. Touch pointers are skipped, so on a phone or a tablet those labels never come up at all, and a tap just runs the action.