Adding automated tests as web platform tests (only for mozilla) for focus move at using Selection API.
Although, there is no standards of relation between Selection API and focus, we should move focus when Selection API moves selections into only an editing host or outside of focused editing host.
Chrome moves focus as this rules, therefore, user can modify contenteditable editor immediately after web app moves selection.
Edge does NOT move focus at using Selection API. However, user can modify contenteditable editor similar to Chrome. We can guess that Edge doesn't need to move focus in its design because perhaps, Edge decides if it's editable only with primary selected range.
We cannot take the Edge behavior due to our editor design. So, we can take only Chrome's approach for improving the compatibility.
MozReview-Commit-ID: JuLiSMgqODm
462139cd3e