Blinking dropdown menu

Hello.
I am a new here with very bad English :face_with_peeking_eye: So, sorry maybe for my incomprehensible question.
I learn to do Dropdown menu, I did everything what I should, but every option while hovering during prototype test is blinking/flashing.
Where could be a problem?
I can not upload attachments,because I am a new user :frowning:

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