fix(template-require-aria-activedescendant-tabindex): accept tabindex="-1"

Before: any tabindex below 0 was flagged, and the autofix replaced
tabindex="-1" with tabindex="0". That silently changed semantics — -1 is
the canonical "focusable but not in tab order" value that composite
widgets with aria-activedescendant specifically want.

tabindex semantics:
  "0"  — focusable, in the natural tab order
  "-1" — focusable programmatically (e.g. via roving focus), skipped in
         tab order

Both are valid for elements that manage focus via aria-activedescendant;
see the W3C APG entry on "Managing focus in composites using
aria-activedescendant":
  https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/#kbd_focus_activedescendant

Matches the check upstream in eslint-plugin-jsx-a11y
(aria-activedescendant-has-tabindex.js: `if (tabIndex >= -1) return;`).
lit-a11y's aria-activedescendant-has-tabindex has the same semantics.

Rule doc updated to describe the new accepted range. Tests moved the
three tabindex="-1" cases from invalid to valid.

Author: johanrd

docs/rules/template-require-aria-activedescendant-tabindex.md

@@ -6,11 +6,11 @@
 
 <!-- end auto-generated rule header -->
 
-This rule requires all non-interactive HTML elements using the `aria-activedescendant` attribute to declare a `tabindex` of zero.
+This rule requires non-interactive HTML elements using the `aria-activedescendant` attribute to declare a `tabindex` of `0` or `-1`.
 
 The `aria-activedescendant` attribute identifies the active descendant element of a composite widget, textbox, group, or application with document focus. This attribute is placed on the container element of the input control, and its value is set to the ID of the active child element. This allows screen readers to communicate information about the currently active element as if it has focus, while actual focus of the DOM remains on the container element.
 
-Elements with `aria-activedescendant` must have a `tabindex` of zero in order to support keyboard navigation. Besides interactive elements, which are inherently keyboard-focusable, elements using the `aria-activedescendant` attribute must declare a `tabIndex` of zero with the `tabIndex` attribute.
+Elements with `aria-activedescendant` must be focusable to support keyboard navigation. `tabindex="0"` puts the element in the natural tab order; `tabindex="-1"` makes it focusable programmatically (e.g. via roving focus) but skips it in the tab order. Both are valid patterns for composite widgets — see the [W3C APG — Managing focus in composites using aria-activedescendant](https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/#kbd_focus_activedescendant).
 
 ## Examples
 
@@ -19,8 +19,8 @@ This rule **forbids** the following:
 ```gjs
 <template>
   <div aria-activedescendant='some-id'></div>
-  <div aria-activedescendant='some-id' tabindex='-1'></div>
-  <input aria-activedescendant={{some-id}} tabindex='-1' />
+  <div aria-activedescendant='some-id' tabindex='-2'></div>
+  <input aria-activedescendant={{some-id}} tabindex='-100' />
 </template>
 ```
 
@@ -32,9 +32,11 @@ This rule **allows** the following:
   <CustomComponent aria-activedescendant={{some-id}} />
   <CustomComponent aria-activedescendant={{some-id}} tabindex={{0}} />
   <div aria-activedescendant='some-id' tabindex='0'></div>
+  <div aria-activedescendant='some-id' tabindex='-1'></div>
   <input />
   <input aria-activedescendant={{some-id}} />
   <input aria-activedescendant={{some-id}} tabindex={{0}} />
+  <input aria-activedescendant={{some-id}} tabindex={{-1}} />
 </template>
 ```
 

lib/rules/template-require-aria-activedescendant-tabindex.js

@@ -91,7 +91,7 @@ module.exports = {
 
         const tabindexValue = getTabindexNumericValue(tabindexAttr);
 
-        if (!Number.isFinite(tabindexValue) || tabindexValue < 0) {
+        if (!Number.isFinite(tabindexValue) || tabindexValue < -1) {
           context.report({
             node,
             messageId: 'missingTabindex',

tests/lib/rules/template-require-aria-activedescendant-tabindex.js

@@ -13,6 +13,12 @@ const validHbs = [
   '<CustomComponent aria-activedescendant="choice1" />',
   '<CustomComponent aria-activedescendant="option1" tabIndex="-1" />',
   '<CustomComponent aria-activedescendant={{foo}} tabindex={{bar}} />',
+  // tabindex="-1" is focusable-but-not-tabbable — the canonical pattern for
+  // composite widgets that manage focus via roving focus / aria-activedescendant.
+  // See W3C APG — https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/#kbd_focus_activedescendant
+  '<div aria-activedescendant="option0" tabindex="-1"></div>',
+  '<div aria-activedescendant={{foo}} tabindex={{-1}}></div>',
+  '<button aria-activedescendant="x" tabindex="-1"></button>',
 ];
 
 const invalidHbs = [
@@ -26,11 +32,6 @@ const invalidHbs = [
     output: '<div aria-activedescendant={{bar}} tabindex="0" />',
     errors: [{ message: ERROR_MESSAGE }],
   },
-  {
-    code: '<div aria-activedescendant={{foo}} tabindex={{-1}}></div>',
-    output: '<div aria-activedescendant={{foo}} tabindex="0"></div>',
-    errors: [{ message: ERROR_MESSAGE }],
-  },
   {
     code: '<div aria-activedescendant="fixme" tabindex=-100></div>',
     output: '<div aria-activedescendant="fixme" tabindex="0"></div>',
@@ -41,11 +42,6 @@ const invalidHbs = [
     output: '<a aria-activedescendant="x" tabindex="0"></a>',
     errors: [{ message: ERROR_MESSAGE }],
   },
-  {
-    code: '<button aria-activedescendant="x" tabindex="-1"></button>',
-    output: '<button aria-activedescendant="x" tabindex="0"></button>',
-    errors: [{ message: ERROR_MESSAGE }],
-  },
 ];
 
 function wrapTemplate(entry) {