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>

scss/_variables.scss

@@ -902,10 +902,11 @@ $form-validation-states: (
 $zindex-dropdown:                   1000 !default;
 $zindex-sticky:                     1020 !default;
 $zindex-fixed:                      1030 !default;
-$zindex-modal-backdrop:             1040 !default;
-$zindex-modal:                      1050 !default;
-$zindex-popover:                    1060 !default;
-$zindex-tooltip:                    1070 !default;
+$zindex-offcanvas:                  1040 !default;
+$zindex-modal-backdrop:             1050 !default;
+$zindex-modal:                      1060 !default;
+$zindex-popover:                    1070 !default;
+$zindex-tooltip:                    1080 !default;
 // scss-docs-end zindex-stack
 
 
@@ -1431,6 +1432,23 @@ $btn-close-white-filter:     invert(1) grayscale(100%) brightness(200%) !default
 // scss-docs-end close-variables
 
 
+// Offcanvas
+
+// scss-docs-start offcanvas-variables
+$offcanvas-padding-y:               $modal-inner-padding !default;
+$offcanvas-padding-x:               $modal-inner-padding !default;
+$offcanvas-horizontal-width:        400px !default;
+$offcanvas-vertical-height:         30vh !default;
+$offcanvas-transition-duration:     .3s !default;
+$offcanvas-border-color:            $modal-content-border-color !default;
+$offcanvas-border-width:            $modal-content-border-width !default;
+$offcanvas-title-line-height:       $modal-title-line-height !default;
+$offcanvas-bg-color:                $modal-content-bg !default;
+$offcanvas-color:                   $modal-content-color !default;
+$offcanvas-body-backdrop-color:     rgba($modal-backdrop-bg, $modal-backdrop-opacity) !default;
+$offcanvas-box-shadow:              $modal-content-box-shadow-xs !default;
+// scss-docs-end offcanvas-variables
+
 // Code
 
 $code-font-size:                    $small-font-size !default;