Fix double/triple click selection issues on headings (#8015)

Co-authored-by: Chris Swithinbank <357379+delucis@users.noreply.github.com>
Co-authored-by: Reuben Tier <64310361+TheOtterlord@users.noreply.github.com>
Co-authored-by: Sarah Rainsberger <5098874+sarah11918@users.noreply.github.com>

src/components/starlight/MarkdownContent.astro

@@ -166,6 +166,12 @@ const { entry } = Astro.props;
 		width: var(--icon-size);
 	}
 
+	/* Prevent double or triple clicks from potentially selecting the anchor link a11y text. */
+	.sl-markdown-content :global(.anchor-link .sr-only) {
+		-webkit-user-select: none;
+		user-select: none;
+	}
+
 	@media (hover: hover) {
 		.sl-markdown-content :global(.anchor-link) {
 			opacity: 0;