Add `.modal-dialog-centered` for optional vertically centered modal (#24510)

* Add .modal-dialog-centered for optional vertically cenetered modal

Fixes #23638

* adds modal-dialog-centered class to docs and removes margin to avoid generating a vertical scrolling

* mention limitations

* fix aria attr

* Add `width: 100%` to the .modal-content for the centered version. Adding it here to avoid adding another selector by limiting it to the centered modal modifier.

docs/4.0/components/modal.md

@@ -208,6 +208,64 @@ When modals become too long for the user's viewport or device, they scroll indep
 </div>
 {% endhighlight %}
 
+### Vertically centered
+
+Add `.modal-dialog-centered` to `.modal-dialog` to vertically center the modal. **Do not use this with long modals**—it will overflow the viewport and potentially hide parts of your modal.
+
+<div id="exampleModalCenter" class="modal fade" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
+  <div class="modal-dialog modal-dialog-centered" role="document">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h5 class="modal-title" id="exampleModalCenterTitle">Modal title</h5>
+        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
+          <span aria-hidden="true">&times;</span>
+        </button>
+      </div>
+      <div class="modal-body">
+        <p>Cras mattis consectetur purus sit amet fermentum. Cras justo odio, dapibus ac facilisis in, egestas eget quam. Morbi leo risus, porta ac consectetur ac, vestibulum at eros.</p>
+      </div>
+      <div class="modal-footer">
+        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
+        <button type="button" class="btn btn-primary">Save changes</button>
+      </div>
+    </div>
+  </div>
+</div>
+
+<div class="bd-example">
+  <button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalCenter">
+    Launch demo modal
+  </button>
+</div>
+
+{% highlight html %}
+<!-- Button trigger modal -->
+<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalCenter">
+  Launch demo modal
+</button>
+
+<!-- Modal -->
+<div class="modal fade" id="exampleModalCenter" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
+  <div class="modal-dialog" role="document">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h5 class="modal-title" id="exampleModalLongTitle">Modal title</h5>
+        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
+          <span aria-hidden="true">&times;</span>
+        </button>
+      </div>
+      <div class="modal-body">
+        ...
+      </div>
+      <div class="modal-footer">
+        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
+        <button type="button" class="btn btn-primary">Save changes</button>
+      </div>
+    </div>
+  </div>
+</div>
+{% endhighlight %}
+
 ### Tooltips and popovers
 
 [Tooltips]({{ site.baseurl }}/docs/{{ site.docs_version }}/components/tooltips/) and [popovers]({{ site.baseurl }}/docs/{{ site.docs_version }}/components/popovers/) can be placed within modals as needed. When modals are closed, any tooltips and popovers within are also automatically dismissed.

scss/_modal.scss

@@ -50,11 +50,20 @@
   }
 }
 
+.modal-dialog-centered {
+  display: flex;
+  align-items: center;
+  height: 100%;
+  margin-top: 0;
+  margin-bottom: 0;
+}
+
 // Actual modal
 .modal-content {
   position: relative;
   display: flex;
   flex-direction: column;
+  width: 100%; // Ensure `.modal-content` extends the full width of the parent `.modal-dialog`
   // counteract the pointer-events: none; in the .modal-dialog
   pointer-events: auto;
   background-color: $modal-content-bg;
@@ -144,11 +153,17 @@
     margin: $modal-dialog-margin-y-sm-up auto;
   }
 
+  .modal-dialog-centered {
+    margin-top: 0;
+    margin-bottom: 0;
+  }
+
   .modal-content {
     @include box-shadow($modal-content-box-shadow-sm-up);
   }
 
   .modal-sm { max-width: $modal-sm; }
+
 }
 
 @include media-breakpoint-up(lg) {