Submenu dropdown's class: .nav-item.dropdown>.navbar-nav.in

[sheralimuzaffarov]

in bsnav.css code:
.nav-item.dropdown>.navbar-nav.in {
pointer-events: initial;
clip: rect(0,50vw,2000px,0);
transition: 1.5s ease-in;
}
but it's cropping like this:

I changed clip property and then it worked:
.nav-item.dropdown>.navbar-nav.in {
clip: rect(0,100vw,2000px,0); // or (clip: unset;)
}
My question: why clip property is needed here?

[fitodac]

Clip is used to show animations for submenus with css instead javascript