Sort Youtube Playlist by Duration (formerly Sort Youtube Watch Later by Duration)
If your playlist is not Watch Later, consider YTPlaylistSorter by CarterRJ first unless it doesn't work or you have privacy concerns. It is faster, more efficient, and doesn't require you to keep that particular playlist tab on for 2 hours at a time.
Youtube Watch Later playlist has always lacked a sort-by-duration function. Without external API access, services like YTPlaylistSorter from CarterRJ don't work either, thus introducing this code, now migrated to work on TamperMonkey/GreaseMonkey/ViolentMonkey.
The original code was gratefully contributed by SunnyWiz, but it was broken when it got to me. I just fixed it and added some features to make it easier to use.
As of v2, the code has gone through a significant rewrite to increase maintainability and bring it up to modern standards.
v3 brings official support to generic playlists. (Previously it worked, but support was not guaranteed.)
After adding this script, additional UI will be appended as shown below.
You can find the code to this script here, and submitting feedback at the Github Gist will lead to faster responses.