Keyframes animation has weird gray line that appears during transition when drop-down button is clicked

Add-ons
1

I am making a drop-down menu with several sections that looks like this: drop down menu that I have added an animation effect that fades in the animation content when the drop-down button is clicked.

2

Could you provide code that you are using to create the drop-down and transition? That would help us be able to help you out.