Scrollable Menu with Bootstrap - Menu expanding its container when it should not

Bootstrap 5 (update 2021)

The dropdown markup has changed for BS 5 as the data- attributes have changed to data-bs-. However, setting max-height still works to make the dropdown scrollable...

.dropdown-menu {
    max-height: 280px;
    overflow-y: auto;
}

https://codeply.com/p/shJzHGE84z

Bootstrap 4 (update 2018)

The dropdown markup has changed for BS 4 as the items have their own dropdown-item class. However, setting max-height still works to make the dropdown scrollable...

.dropdown-menu {
    max-height: 280px;
    overflow-y: auto;
}

Bootstrap scrollable dropdown

Alternative horizontal menu for Bootstrap 4 using flexbox

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Bootstrap 3 (original answer)

I think you can simplify this by just adding the necessary CSS properties to your special scrollable menu class..

CSS:

.scrollable-menu {
    height: auto;
    max-height: 200px;
    overflow-x: hidden;
}

HTML

       <ul class="dropdown-menu scrollable-menu" role="menu">
          <li><a href="#">Action</a></li>
          <li><a href="#">Another action</a></li>
          <li><a href="#">Something else here</a></li>
          <li><a href="#">Action</a></li>
            ..
          <li><a href="#">Action</a></li>
          <li><a href="#">Another action</a></li>
       </ul>

Working example: https://codeply.com/p/ox7JC49vmT

For CSS, I found that max height of 180 is better for mobile phones landscape 320 when showing browser chrome.

.scrollable-menu {
    height: auto;
    max-height: 180px;
    overflow-x: hidden;
}

Also, to add visible scrollbars, this CSS should do the trick:

.scrollable-menu::-webkit-scrollbar {
    -webkit-appearance: none;
    width: 4px;        
}    
.scrollable-menu::-webkit-scrollbar-thumb {
    border-radius: 3px;
    background-color: lightgray;
    -webkit-box-shadow: 0 0 1px rgba(255,255,255,.75);        
}

The changes are reflected here: https://www.bootply.com/BhkCKFEELL

You can use the built-in CSS class pre-scrollable in bootstrap 3 inside the span element of the dropdown and it works immediately without implementing custom css.

 <ul class="dropdown-menu pre-scrollable">
                <li>item 1 </li>
                <li>item 2 </li>

 </ul>

Do everything in the inline of UL tag

<ul class="dropdown-menu scrollable-menu" role="menu" style="height: auto;max-height: 200px; overflow-x: hidden;">
                <li><a href="#">Action</a></li>
                <li><a href="#">Another action</a></li>
                <li><a href="#">Something else here</a></li>
                <li><a href="#">Action</a></li>
                ..
                <li><a href="#">Action</a></li>
                <li><a href="#">Another action</a></li>
            </ul>