How to write a code that can show a hidden menu bar when mouse come over on it ?

Answer 1

You can use events like onmouseenter and onmouseleave and then use js to toggle css visibility or make a transition.

Answer 2

Reference this:
https://code.sololearn.com/W7v5Pmo7E183/?ref=app
When an element is on hover, it makes the list height increase.