fix: keep the em/strong mask the same length as the source

src/Lexer.ts

@@ -316,8 +316,11 @@ export class _Lexer<ParserOutput = string, RendererOutput = string> {
       }
     }
 
-    // Mask out escaped characters
-    maskedSrc = maskedSrc.replace(this.tokenizer.rules.inline.anyPunctuation, '++');
+    // Mask out escaped characters.
+    // Every mask must keep the length it replaces: emStrong and del line
+    // maskedSrc up with src by slicing from the end. `anyPunctuation` matches
+    // unicode punctuation, so an escaped astral character is 3 code units.
+    maskedSrc = maskedSrc.replace(this.tokenizer.rules.inline.anyPunctuation, match0 => '+'.repeat(match0.length));
 
     // Mask out other blocks
     maskedSrc = maskedSrc.replace(this.tokenizer.rules.inline.blockSkip, (match0, _link, context) => {

test/specs/new/em_escaped_astral_punctuation.html

@@ -0,0 +1,7 @@
+<p><em>a</em>\🙂</p>
+<p><em>a</em> \🙂 <em>b</em></p>
+<p>x<em>ab</em>\🙂y</p>
+<p><em>a</em>a\𐄀<em>b</em></p>
+<p><strong>a</strong>\🙂b</p>
+<p><del>a</del>\🙂</p>
+<p><em>foo\🙂bar</em></p>

test/specs/new/em_escaped_astral_punctuation.md

@@ -0,0 +1,16 @@
+---
+gfm: true
+---
+*a*\🙂
+
+*a* \🙂 *b*
+
+x*ab*\🙂y
+
+*a*a\𐄀*b*
+
+__a__\🙂b
+
+~~a~~\🙂
+
+*foo\🙂bar*