Firefox doesn't implement the IE6 fromElement/toElement, and it's
not in the MouseEvent spec (at the moment).
Replace with the worse-named but better-specified target and
relatedTarget attributes instead.
Upstream change: https://gerrit-review.googlesource.com/q/I9eeb26c032a38de9d7185749373c7982c796acb2
Change-Id: I9f9a1eb9342bc80b91b5b364a04cc5fa9a7ccaeb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1442
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
