Assumption: You use YouTube's watch later list not as a real playlist but to collect interesting (and not necessarily related) videos.
Problem: When you start a single video from the playlist page, the following videos (related or not) will be played too, automatically.
Old solution: An extension called NextVid Stopper which does not work anymore.
New solution: This extension ;) It redirects the start of a single video from the watch later page to the single video player instead of the playlist player. You can still click the "Play All" button to play the list as a whole.
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History:
v0.2: The single play behaviour can be configured for all playlists or for watch lists only.
v0.1: Initial release, works only for the watch list.
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Known problems:
If you have the watch later list already playing in one tab and, in another tab, open a single video from the watch later list YouTube somehow makes an additional redirect and you get two entries in the browser history.
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Required Permissions:
"storage" to save the options mode to chrome
"tabs" to verify that a request comes from the watch later list
"webRequest", "webRequestBlocking" to redirect the request to the correct player