Add dropdown responsive alignment (#26255)

Author: ysds

scss/_dropdown.scss

@@ -33,9 +33,26 @@
   @include box-shadow($dropdown-box-shadow);
 }
 
-.dropdown-menu-right {
-  right: 0;
-  left: auto;
+@each $breakpoint in map-keys($grid-breakpoints) {
+  @include media-breakpoint-up($breakpoint) {
+    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);
+
+    .dropdown-menu#{$infix}-right {
+      right: 0;
+      left: auto;
+    }
+  }
+}
+
+@each $breakpoint in map-keys($grid-breakpoints) {
+  @include media-breakpoint-up($breakpoint) {
+    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);
+
+    .dropdown-menu#{$infix}-left {
+      right: auto;
+      left: 0;
+    }
+  }
 }
 
 // Allow for dropdowns to go bottom up (aka, dropup-menu)

site/docs/4.1/components/dropdowns.md

@@ -616,6 +616,44 @@ By default, a dropdown menu is automatically positioned 100% from the top and al
 {% endcapture %}
 {% include example.html content=example %}
 
+### Responsive alignment
+
+If you want to use responsive alignment, disable dynamic positioning by adding the `data-display="static"` attribute and use the responsive variation classes.
+
+To align **right** the dropdown menu with the given breakpoint or larger, add `.dropdown-menu{-sm|-md|-lg|-xl}-right`.
+
+{% capture example %}
+<div class="btn-group">
+  <button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" data-display="static" aria-haspopup="true" aria-expanded="false">
+    Left-aligned but right aligned when large screen
+  </button>
+  <div class="dropdown-menu dropdown-menu-lg-right">
+    <button class="dropdown-item" type="button">Action</button>
+    <button class="dropdown-item" type="button">Another action</button>
+    <button class="dropdown-item" type="button">Something else here</button>
+  </div>
+</div>
+{% endcapture %}
+{% include example.html content=example %}
+
+To align **left** the dropdown menu with the given breakpoint or larger, add `.dropdown-menu-right` and `.dropdown-menu{-sm|-md|-lg|-xl}-left`.
+
+{% capture example %}
+<div class="btn-group">
+  <button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" data-display="static" aria-haspopup="true" aria-expanded="false">
+    Right-aligned but left aligned when large screen
+  </button>
+  <div class="dropdown-menu dropdown-menu-right dropdown-menu-lg-left">
+    <button class="dropdown-item" type="button">Action</button>
+    <button class="dropdown-item" type="button">Another action</button>
+    <button class="dropdown-item" type="button">Something else here</button>
+  </div>
+</div>
+{% endcapture %}
+{% include example.html content=example %}
+
+Note that you don't need to add a `data-display="static"` attribute to dropdown buttons in navbars, since Popper.js isn't used in navbars.
+
 ## Menu content
 
 ### Headers