Add centered dropdown and dropup options

Author: mdo

js/src/dropdown.js

@@ -48,6 +48,8 @@ const CLASS_NAME_SHOW = 'show'
 const CLASS_NAME_DROPUP = 'dropup'
 const CLASS_NAME_DROPEND = 'dropend'
 const CLASS_NAME_DROPSTART = 'dropstart'
+const CLASS_NAME_DROPUP_CENTER = 'dropup-center'
+const CLASS_NAME_DROPDOWN_CENTER = 'dropdown-center'
 
 const SELECTOR_DATA_TOGGLE = '[data-bs-toggle="dropdown"]:not(.disabled):not(:disabled)'
 const SELECTOR_DATA_TOGGLE_SHOWN = `${SELECTOR_DATA_TOGGLE}.${CLASS_NAME_SHOW}`
@@ -62,6 +64,8 @@ const PLACEMENT_BOTTOM = isRTL() ? 'bottom-end' : 'bottom-start'
 const PLACEMENT_BOTTOMEND = isRTL() ? 'bottom-start' : 'bottom-end'
 const PLACEMENT_RIGHT = isRTL() ? 'left-start' : 'right-start'
 const PLACEMENT_LEFT = isRTL() ? 'right-start' : 'left-start'
+const PLACEMENT_TOPCENTER = 'top'
+const PLACEMENT_BOTTOMCENTER = 'bottom'
 
 const Default = {
   offset: [0, 2],
@@ -248,6 +252,14 @@ class Dropdown extends BaseComponent {
       return PLACEMENT_LEFT
     }
 
+    if (parentDropdown.classList.contains(CLASS_NAME_DROPUP_CENTER)) {
+      return PLACEMENT_TOPCENTER
+    }
+
+    if (parentDropdown.classList.contains(CLASS_NAME_DROPDOWN_CENTER)) {
+      return PLACEMENT_BOTTOMCENTER
+    }
+
     // We need to trim the value because custom properties can also include spaces
     const isEnd = getComputedStyle(this._menu).getPropertyValue('--bs-position').trim() === 'end'
 

scss/_dropdown.scss

@@ -2,7 +2,9 @@
 .dropup,
 .dropend,
 .dropdown,
-.dropstart {
+.dropstart,
+.dropup-center,
+.dropdown-center {
   position: relative;
 }
 

site/content/docs/5.1/components/dropdowns.md

@@ -400,6 +400,23 @@ And putting it to use in a navbar:
 Directions are mirrored when using Bootstrap in RTL, meaning `.dropstart` will appear on the right side.
 {{< /callout >}}
 
+### Centered
+
+Make the dropdown menu centered below the toggle with `.dropdown-center` on the parent element.
+
+{{< example >}}
+<div class="dropdown-center">
+  <button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownCenterBtn" data-bs-toggle="dropdown" aria-expanded="false">
+    Centered dropdown
+  </button>
+  <ul class="dropdown-menu" aria-labelledby="dropdownCenterBtn">
+    <li><a class="dropdown-item" href="#">Action</a></li>
+    <li><a class="dropdown-item" href="#">Action two</a></li>
+    <li><a class="dropdown-item" href="#">Action three</a></li>
+  </ul>
+</div>
+{{< /example >}}
+
 ### Dropup
 
 Trigger dropdown menus above elements by adding `.dropup` to the parent element.
@@ -459,6 +476,23 @@ Trigger dropdown menus above elements by adding `.dropup` to the parent element.
 </div>
 ```
 
+### Dropup centered
+
+Make the dropup menu centered above the toggle with `.dropup-center` on the parent element.
+
+{{< example >}}
+<div class="dropup-center">
+  <button class="btn btn-secondary dropdown-toggle" type="button" id="dropupCenterBtn" data-bs-toggle="dropdown" aria-expanded="false">
+    Centered dropup
+  </button>
+  <ul class="dropdown-menu" aria-labelledby="dropupCenterBtn">
+    <li><a class="dropdown-item" href="#">Action</a></li>
+    <li><a class="dropdown-item" href="#">Action two</a></li>
+    <li><a class="dropdown-item" href="#">Action three</a></li>
+  </ul>
+</div>
+{{< /example >}}
+
 ### Dropend
 
 Trigger dropdown menus at the right of the elements by adding `.dropend` to the parent element.