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Moves the controls under the video and makes the UI look like it was before august 2015
What exactly happens when you try to access this sub-menu? A screenshot can be useful.
It seems to work on my end, tried using multiple browser (including waterfox): 
@Roy192 said: What exactly happens when you try to access this sub-menu? A screenshot can be useful.
It seems to work on my end, tried using multiple browser (including waterfox): The sub-menu wouldn't show while clicking on it. After the more complete test, I found this problem was caused by another scripts: RemoveAds. Enabling both of scripts would cause this, but separately enabling either "RemoveAds" or "Youtube UI Fix" won't result in this issue. Please inform the users that do not use them together.
The description has now been edited to include an incompatibilities section.
Though I still haven't been able to reproduce it...
@Roy192 said: The description has now been edited to include an incompatibilities section.
Though I still haven't been able to reproduce it... The issue I encountered wasn't that the vertical 3 of points didn't appear, but clicked on it wouldn't show any menu.
A glitch about the sub-menu of video
When hovering over the preview screenshot of video, there's a sub-menu which can add a video to a playlist, reporting an inappropriate video or other functions. The sub-menu looks like vertical 3 of points.
If enabling the scripts, this sub-menu won't work.