new linkify handling

Should fix mentions in non-HTML messages

lib/helpers/extensions/linkify_html.dart

@@ -0,0 +1,45 @@
+import "package:html/dom.dart";
+import "package:html/parser.dart";
+import "package:linkify/linkify.dart";
+import "package:nexus/helpers/matrix_linkifier.dart";
+
+extension LinkifyHtml on String {
+  /// Wraps bare links in this HTML's text in `<a>` tags.
+  ///
+  /// Only text nodes are touched, so attribute values are left alone, and
+  /// text already inside a link or code is skipped.
+  String get linkifiedHtml {
+    final fragment = parseFragment(this);
+    _linkifyNode(fragment);
+    return fragment.outerHtml;
+  }
+}
+
+void _linkifyNode(Node node) {
+  if (node is Element && const {"a", "code", "pre"}.contains(node.localName)) {
+    return;
+  }
+
+  for (final child in node.nodes.toList()) {
+    if (child is! Text) {
+      _linkifyNode(child);
+      continue;
+    }
+
+    final elements = linkifyText(child.data);
+    if (!elements.any((element) => element is LinkableElement)) continue;
+
+    elements
+        .map(
+          (element) => switch (element) {
+            LinkableElement(:final text, :final url) =>
+              Element.tag("a")
+                ..attributes["href"] = url
+                ..append(Text(text)),
+            _ => Text(element.text),
+          },
+        )
+        .forEach((replacement) => node.insertBefore(replacement, child));
+    child.remove();
+  }
+}

lib/helpers/matrix_linkifier.dart

@@ -0,0 +1,38 @@
+import "package:linkify/linkify.dart";
+
+/// Splits [text] into plain text and URL, `matrix:` and email links.
+List<LinkifyElement> linkifyText(String text) => linkify(
+  text,
+  options: const LinkifyOptions(humanize: false),
+  linkifiers: const [UrlLinkifier(), MatrixLinkifier(), EmailLinkifier()],
+);
+
+/// Linkifies `matrix:` URIs, which [UrlLinkifier] only handles for http(s).
+final class const MatrixLinkifier() extends Linkifier {
+  static final _regex = RegExp(
+    r"\bmatrix:[^\s<>\x22\x27]*[^\s<>\x22\x27.,;:!?)\]}]",
+    caseSensitive: false,
+  );
+
+  @override
+  List<LinkifyElement> parse(
+    List<LinkifyElement> elements,
+    LinkifyOptions options,
+  ) => elements
+      .expand(
+        (element) => element is TextElement ? _split(element.text) : [element],
+      )
+      .toList();
+
+  Iterable<LinkifyElement> _split(String text) sync* {
+    var start = 0;
+    for (final match in _regex.allMatches(text)) {
+      if (match.start > start) {
+        yield TextElement(text.substring(start, match.start));
+      }
+      yield UrlElement(match[0]!);
+      start = match.end;
+    }
+    if (start < text.length) yield TextElement(text.substring(start));
+  }
+}