Offcanvas as component (#29017)

* Add a new offcanvas component

* offcanvas.js: switch to string constants and `event.key`

* Remove unneeded code

* Sass optimizations

* Fixes

Make sure the element is hidden and not offscreen when inactive
fix close icon negative margins
Add content in right & bottom examples
Re-fix bottom offcanvas height not to cover all viewport

* Wording tweaks

* update tests and offcanvas class

* separate scrollbar functionality and use it in offcanvas

* Update .bundlewatch.config.json

* fix focus

* update btn-close / fix focus on close

* add aria-modal and role
return focus on trigger when offcanvas is closed
change body scrolling timings

* move common code to reusable functions

* add aria-labelledby

* Replace lorem ipsum text

* fix focus when offcanvas is closed

* updates

* revert modal, add tests for scrollbar

* show backdrop by default

* Update offcanvas.md

* Update offcanvas CSS to better match modals

- Add background-clip for borders
- Move from outline to border (less clever, more consistent)
- Add scss-docs in vars

* Revamp offcanvas docs

- Add static example to show and explain the components
- Split live examples and rename them
- Simplify example content
- Expand docs notes elsewhere
- Add sass docs

* Add .offcanvas-title instead of .modal-title

* Rename offcanvas example to offcanvas-navbar to reflect it's purpose

* labelledby references title and not header

* Add default shadow to offcanvas

* enable offcanvas-body to fill all the remaining wrapper area

* Be more descriptive, on Accessibility area

* remove redundant classes

* ensure in case of an already open offcanvas, not to open another one

* bring back backdrop|scroll combinations

* bring back toggling class

* refactor scrollbar method, plus tests

* add check if element is not full-width, according to #30621

* revert all in modal

* use documentElement innerWidth

* Rename classes to -start and -end

Also copyedit some docs wording

* omit some things on scrollbar

* PASS BrowserStack tests

-- IOS devices, Android devices and Browsers on Mac, hide scrollbar by default and appear it, only while scrolling.

* Rename '_handleClosing' to '_addEventListeners'

* change pipe usage to comma

* change Data.getData to Data.get

Co-authored-by: XhmikosR <xhmikosr@gmail.com>
Co-authored-by: Martijn Cuppens <martijn.cuppens@gmail.com>
Co-authored-by: Mark Otto <markdotto@gmail.com>

site/content/docs/5.0/examples/offcanvas-navbar/offcanvas.css

@@ -0,0 +1,67 @@
+html,
+body {
+  overflow-x: hidden; /* Prevent scroll on narrow devices */
+}
+
+body {
+  padding-top: 56px;
+}
+
+@media (max-width: 991.98px) {
+  .offcanvas-collapse {
+    position: fixed;
+    top: 56px; /* Height of navbar */
+    bottom: 0;
+    left: 100%;
+    width: 100%;
+    padding-right: 1rem;
+    padding-left: 1rem;
+    overflow-y: auto;
+    visibility: hidden;
+    background-color: #343a40;
+    transition: transform .3s ease-in-out, visibility .3s ease-in-out;
+  }
+  .offcanvas-collapse.open {
+    visibility: visible;
+    transform: translateX(-100%);
+  }
+}
+
+.nav-scroller {
+  position: relative;
+  z-index: 2;
+  height: 2.75rem;
+  overflow-y: hidden;
+}
+
+.nav-scroller .nav {
+  display: flex;
+  flex-wrap: nowrap;
+  padding-bottom: 1rem;
+  margin-top: -1px;
+  overflow-x: auto;
+  color: rgba(255, 255, 255, .75);
+  text-align: center;
+  white-space: nowrap;
+  -webkit-overflow-scrolling: touch;
+}
+
+.nav-underline .nav-link {
+  padding-top: .75rem;
+  padding-bottom: .75rem;
+  font-size: .875rem;
+  color: #6c757d;
+}
+
+.nav-underline .nav-link:hover {
+  color: #007bff;
+}
+
+.nav-underline .active {
+  font-weight: 500;
+  color: #343a40;
+}
+
+.text-white-50 { color: rgba(255, 255, 255, .5); }
+
+.bg-purple { background-color: #6f42c1; }