@@ -0,0 +1,15 @@
+// Carries the page's `feature` query param over to a link while keeping the link's own query and fragment.
+export function addFeatureParamToHref(href: string, search: string): string {
+  const feature = new URLSearchParams(search).get('feature')
+  // In-page fragment links never reach the server, so they gain nothing from the param.
+  if (!feature || !href || href.startsWith('#')) return href
+
+  const hashIndex = href.indexOf('#')
+  const beforeHash = hashIndex === -1 ? href : href.slice(0, hashIndex)
+  const hash = hashIndex === -1 ? '' : href.slice(hashIndex)
+  const queryIndex = beforeHash.indexOf('?')
+  const path = queryIndex === -1 ? beforeHash : beforeHash.slice(0, queryIndex)
+  const params = new URLSearchParams(queryIndex === -1 ? '' : beforeHash.slice(queryIndex + 1))
+  params.set('feature', feature)
+  return `${path}?${params.toString()}${hash}`
+}
@@ -11,6 +11,7 @@ import { useTranslation } from '@/languages/components/useTranslation'
 import useCopyClipboard from '@/rest/components/useClipboard'
 import { EventType } from '@/events/types'
 import { sendEvent } from '@/events/components/events'
+import { addFeatureParamToHref } from '@/frame/components/lib/add-feature-param-to-href'
 
 import styles from './MarkdownContent.module.scss'
 
@@ -94,22 +95,9 @@ export const UnrenderedMarkdownContent = ({
 
   // Custom anchors preserve feature query params and can force links into new tabs.
   components.a = ({ ...props }) => {
-    let href = props.href || ''
-    let existingAnchorParams = ''
-    if (includeQueryParams) {
-      if (href.includes('?')) {
-        href = href.split('?')[0]
-        existingAnchorParams = href.split('?')[1]
-      }
-      const existingURLParams = new URLSearchParams(window.location.search)
-      const newParams = new URLSearchParams()
-      if (existingURLParams.get('feature')) {
-        newParams.set('feature', existingURLParams.get('feature') || '')
-      }
-      if (newParams.toString()) {
-        href = `${href}?${existingAnchorParams}&${newParams.toString()}`
-      }
-    }
+    const href = includeQueryParams
+      ? addFeatureParamToHref(props.href || '', window.location.search)
+      : props.href || ''
     return (
       <a
         {...props}
 
@@ -0,0 +1,51 @@
+import { describe, expect, test } from 'vitest'
+
+import { addFeatureParamToHref } from '@/frame/components/lib/add-feature-param-to-href'
+
+const restLink =
+  'https://docs.github.com/en/rest/repos/repos?apiVersion=2022-11-28#create-a-repository'
+
+describe('addFeatureParamToHref', () => {
+  test('keeps query and fragment when there is no feature param', () => {
+    expect(addFeatureParamToHref(restLink, '')).toBe(restLink)
+    expect(addFeatureParamToHref(restLink, '?foo=bar')).toBe(restLink)
+  })
+
+  test('adds feature param after the existing query and before the fragment', () => {
+    expect(addFeatureParamToHref(restLink, '?feature=foo')).toBe(
+      'https://docs.github.com/en/rest/repos/repos?apiVersion=2022-11-28&feature=foo#create-a-repository',
+    )
+  })
+
+  test('adds feature param before the fragment when there is no query', () => {
+    expect(addFeatureParamToHref('https://docs.github.com/en/foo#section', '?feature=foo')).toBe(
+      'https://docs.github.com/en/foo?feature=foo#section',
+    )
+  })
+
+  test('adds feature param to a plain link', () => {
+    expect(addFeatureParamToHref('https://docs.github.com/en/foo', '?feature=foo')).toBe(
+      'https://docs.github.com/en/foo?feature=foo',
+    )
+  })
+
+  test('keeps relative links relative', () => {
+    expect(addFeatureParamToHref('/en/foo?tool=cli', '?feature=foo')).toBe(
+      '/en/foo?tool=cli&feature=foo',
+    )
+  })
+
+  test('replaces an existing feature param instead of duplicating it', () => {
+    expect(
+      addFeatureParamToHref('https://docs.github.com/en/foo?feature=old&tool=cli', '?feature=foo'),
+    ).toBe('https://docs.github.com/en/foo?feature=foo&tool=cli')
+  })
+
+  test('leaves in-page fragment links unchanged', () => {
+    expect(addFeatureParamToHref('#section', '?feature=foo')).toBe('#section')
+  })
+
+  test('leaves an empty href empty', () => {
+    expect(addFeatureParamToHref('', '?feature=foo')).toBe('')
+  })
+})