It is due to media query used in Bootstrap!
You need to learn media queries for that or if you don't need media queries! Don't use classes of Bootstrap in navigation!
Put this into @media (min-width: 768px) and (max-width: 979px) and it works perfectly.
.nav-collapse, .nav-collapse.collapse {
overflow: visible;
}
.navbar .btn-navbar {
display: none;
}
or
You can also stop the navbar from stacking by changing the
@grid-float-breakpoint variable to 1px
or
Media queries works on browser width for mobile devices u can also specify your style in media query css
@media(max-width:767px){}
@media(min-width:768px){}
@media(min-width:992px){}
@media(min-width:1200px){}
Here is link for disabling media queries