fix(tokenizer): require full --> to close comments in xmlMode (#2383)

In HTML, `<!-->` is a valid empty comment per the spec. However, in XML
comments must be closed by `-->`, so `<!-->` should be treated as an
opening — not a complete comment. Skip the short-comment shortcut when
xmlMode is enabled.

Fixes #1823

Author: fb55

src/Tokenizer.spec.ts

@@ -158,6 +158,15 @@ describe("Tokenizer", () => {
         ).toMatchSnapshot();
     });
 
+    it("should not treat <!-->  as a complete comment in xmlMode", () => {
+        expect(
+            tokenize(
+                "<root><node>start</node><!--><node>should ignore</node><--><node>end</node></root>",
+                { xmlMode: true },
+            ),
+        ).toMatchSnapshot();
+    });
+
     it.each([
         "script",
         "style",

src/Tokenizer.ts

@@ -622,8 +622,11 @@ export default class Tokenizer {
         if (c === CharCodes.Dash) {
             this.state = State.InCommentLike;
             this.currentSequence = Sequences.CommentEnd;
-            // Allow short comments (eg. <!-->)
-            this.sequenceIndex = 2;
+            /*
+             * In HTML, `<!-->` is a valid empty comment. In XML, comments
+             * must be closed by `-->`, so we require the full sequence.
+             */
+            this.sequenceIndex = this.xmlMode ? 0 : 2;
             this.sectionStart = this.index + 1;
         } else {
             this.state = State.InDeclaration;

src/__snapshots__/Tokenizer.spec.ts.snap

@@ -565,6 +565,72 @@ exports[`Tokenizer > should not lose data when pausing 1`] = `
 ]
 `;
 
+exports[`Tokenizer > should not treat <!-->  as a complete comment in xmlMode 1`] = `
+[
+  [
+    "onopentagname",
+    1,
+    5,
+  ],
+  [
+    "onopentagend",
+    5,
+  ],
+  [
+    "onopentagname",
+    7,
+    11,
+  ],
+  [
+    "onopentagend",
+    11,
+  ],
+  [
+    "ontext",
+    12,
+    17,
+  ],
+  [
+    "onclosetag",
+    19,
+    23,
+  ],
+  [
+    "oncomment",
+    28,
+    58,
+    2,
+  ],
+  [
+    "onopentagname",
+    60,
+    64,
+  ],
+  [
+    "onopentagend",
+    64,
+  ],
+  [
+    "ontext",
+    65,
+    68,
+  ],
+  [
+    "onclosetag",
+    70,
+    74,
+  ],
+  [
+    "onclosetag",
+    77,
+    81,
+  ],
+  [
+    "onend",
+  ],
+]
+`;
+
 exports[`Tokenizer > should support self-closing special tags > for self-closing script tag 1`] = `
 [
   [