Updated "Scrollable content can be reached with sequential focus navigation" [0ssw9k]: scrollable regions pass when there is an open modal (act-rules#2219)

* scrollable regions should pass when there is an open modal

- updated expectation to exclude inert elements from sequential focus
- added passing example

* Update _rules/scrollable-element-keyboard-accessible-0ssw9k.md

Co-authored-by: Jean-Yves Moyen <jym@siteimprove.com>

* Update scrollable-element-keyboard-accessible-0ssw9k.md

---------

Co-authored-by: Jean-Yves Moyen <jym@siteimprove.com>
Co-authored-by: Wilco Fiers <WilcoFiers@users.noreply.github.com>

Author: 3 people

_rules/scrollable-element-keyboard-accessible-0ssw9k.md

@@ -39,7 +39,10 @@ This rule applies to any [HTML element][] that has [visible][] [children][] in t
 
 ## Expectation
 
-Each test target is either included in [sequential focus navigation][] or has a [descendant][] in the [flat tree][] that is included in [sequential focus navigation][].
+For each target element, at least one of the following is true: 
+- the element is included in [sequential focus navigation][]; or 
+- the element has a [descendant][] in the [flat tree][] that is included in [sequential focus navigation][]; or
+- the element is [inert][].
 
 ## Assumptions
 
@@ -108,6 +111,46 @@ This [scrollable][] `section` element contains a link that is included in [seque
 </section>
 ```
 
+#### Passed Example 3
+
+This [scrollable][] `section` element is [inert][] because of the modal dialog, so neither the `section` nor its [descendant][] elements are included in [sequential focus navigation][].
+
+```html
+<style>
+	dialog:-internal-dialog-in-top-layer::backdrop {
+    		background: rgba(1, 1, 1, 0.8);
+	}
+</style>
+<section style="height: 100px; width: 500px; overflow: scroll;" tabindex="0">
+	<h1>WCAG 2.1 Abstract</h1>
+	<p>
+		Web Content Accessibility Guidelines (WCAG) 2.1 covers a wide range of recommendations for making Web content more
+		accessible. Following these guidelines will make content more accessible to a wider range of people with
+		disabilities, including accommodations for blindness and low vision, deafness and hearing loss, limited movement,
+		speech disabilities, photosensitivity, and combinations of these, and some accommodation for learning disabilities
+		and cognitive limitations; but will not address every user need for people with these disabilities. These guidelines
+		address accessibility of web content on desktops, laptops, tablets, and mobile devices. Following these guidelines
+		will also often make Web content more usable to users in general.
+		<button id="ppButton" onclick="openDialog()">Read more about WCAG 2.2</button>
+	</p>
+</section>
+<dialog id="ppDialog" aria-labelledby="dialogLabel">
+	<h2 id="dialogLabel">WCAG 2.2</h2>
+	<p>
+		<a href="https://www.w3.org/TR/WCAG22/">WCAG 2.2</a>
+	</p>
+	<button id="cancel" onclick="ppDialog.close()">Cancel</button>
+</dialog>
+<script>
+	const openDialog = () => {
+		ppDialog.showModal();
+		myFrame.tabIndex = '-1'
+	}
+	ppDialog.addEventListener('close', () => myFrame.tabIndex = 0)
+	window.addEventListener('DOMContentLoaded', openDialog);
+</script>
+```
+
 ### Failed
 
 #### Failed Example 1
@@ -262,6 +305,7 @@ This `iframe` element is not a scrollable element, but instead contains a nested
 [sequential focus navigation]: https://html.spec.whatwg.org/multipage/interaction.html#sequential-focus-navigation 'HTML sequential focus navigation, 2020/04/03'
 [flat tree]: https://drafts.csswg.org/css-scoping/#flat-tree 'CSS draft, flat tree, 2020/04/03'
 [computed]: https://www.w3.org/TR/css-cascade-3/#computed-value
+[inert]: #inert 'Definition of Inert'
 [overflow]: https://www.w3.org/TR/CSS22/visufx.html#overflow
 [padding]: https://www.w3.org/TR/CSS22/box.html#propdef-padding
 [padding-left]: https://www.w3.org/TR/CSS22/box.html#propdef-padding-left