I run Splits.io which helps speedrunners highlight their runs on Twitch by sending them to the Twitch highlight page with the grabbers already set correctly via URL parameters, based on their speedrun timer data (thanks for supplying these URL parameters, they help a lot!).
Since the old highlight page broke,
https://www.twitch.tv/:user/manager/highlighter/:video
and we’ve moved to the new one using the new dashboard,
https://dashboard.twitch.tv/u/:user/content/video-producer/highlighter/:video
passing said URL params (e.g. ?start=100&end=100) causes a JavaScript alert upon pageload that reads:
Your highlights have not been created. Are you sure you want to leave?
This is in a new tab with no highlight in progress, so the alert makes no sense. The URL params still have the intended effect, it just spawns this unrelated JS alert on pageload first. I assume this is related to the URL rewrite from e.g.
/u/glacials/content/video-producer/highlighter/505221289?start=100&end=200
to
/u/glacials/content/video-producer/highlighter/505221289
So right now, best case scenario is we have to instruct users to ignore the bogus JS alert when we send them to the highlight page. 
Thanks for reading!
EDIT: I no longer work at Twitch, but this dev forum still thinks I do! Can someone remove my permissions?