4.3: a bare backtick run escapes wherever a later string of its length forms around an escape

corpus/round-trip/combinations/code-span-joined-run.json

@@ -0,0 +1,73 @@
+{
+  "content": [
+    {
+      "content": [
+        {
+          "text": "`` ``",
+          "type": "text"
+        },
+        {
+          "marks": [
+            {
+              "type": "code"
+            }
+          ],
+          "text": "a",
+          "type": "text"
+        }
+      ],
+      "type": "paragraph"
+    },
+    {
+      "content": [
+        {
+          "marks": [
+            {
+              "type": "underline"
+            }
+          ],
+          "text": "``",
+          "type": "text"
+        },
+        {
+          "text": "``",
+          "type": "text"
+        },
+        {
+          "marks": [
+            {
+              "type": "code"
+            }
+          ],
+          "text": "a",
+          "type": "text"
+        }
+      ],
+      "type": "paragraph"
+    },
+    {
+      "content": [
+        {
+          "text": "``\\`",
+          "type": "text"
+        },
+        {
+          "marks": [
+            {
+              "type": "code"
+            }
+          ],
+          "text": "`",
+          "type": "text"
+        },
+        {
+          "text": "`",
+          "type": "text"
+        }
+      ],
+      "type": "paragraph"
+    }
+  ],
+  "type": "doc",
+  "version": 1
+}

corpus/round-trip/combinations/code-span-joined-run.md

@@ -0,0 +1,5 @@
+\`\` \`\``a`
+
+:underline[\`\`]\`\``a`
+
+\`\`\\\``` ` ``\`

spec/flavour.md

@@ -54,8 +54,10 @@ normalizes to it through the round-trip.
 - Entity references in input decode to their characters; output backslash-escapes only where text
   would otherwise parse as syntax, scanning the assembled line rather than each text node: escape
   the leading delimiter of a construct that would otherwise open, re-scan from there, and repeat.
-  A backtick run escapes whole, and a lone backtick escapes wherever an escaped one follows it in
-  the same inline content: CommonMark reads no escape inside a code span, so `` \` `` closes one.
+  A backtick run escapes whole, and a bare one escapes wherever a later backtick string of its
+  length forms around an escape in the same inline content — an escaped backtick alone, one joined
+  to the bare run after it, or a bare run an escape splits off: CommonMark reads no escape inside a
+  code span, so such a string still closes one.
   Where a paragraph opens with what reads as a link reference definition, which resolves before
   any inline construct binds (a `]` inside a code span or `{attrs}` counts), an opening text `[`
   escapes and an opening link's nodes ride the carry.

src/markdown/emit/line-escaping.ts

@@ -103,17 +103,33 @@ function escapedIndexes(scan: string, escapings: readonly InlineEscaping[], cont
       escaped.add(index)
     }
   }
-  escapeLoneBackticks(scan, escapings, escaped)
+  escapeClosedRuns(scan, escapings, escaped)
   return escaped
 }
 
-// CommonMark reads no escape inside a code span, so an escaped backtick still closes one a bare lone backtick before it opens.
-function escapeLoneBackticks(scan: string, escapings: readonly InlineEscaping[], escaped: Set<number>): void {
-  let escapedAfter = false
-  for (let index = scan.length - 1; index >= 0; index -= 1) {
-    if (scan.charAt(index) !== '`') continue
-    if (escapedAfter && escapings[index] !== 'none' && scan.charAt(index - 1) !== '`' && scan.charAt(index + 1) !== '`') escaped.add(index)
-    if (escaped.has(index)) escapedAfter = true
+// CommonMark reads no escape inside a code span, so a backtick string an escape forms or splits off still closes one an earlier bare run opens.
+function escapeClosedRuns(scan: string, escapings: readonly InlineEscaping[], escaped: Set<number>): void {
+  const formed = new Set<number>()
+  let end = scan.length - 1
+  while (end >= 0) {
+    if (scan.charAt(end) !== '`') {
+      end -= 1
+      continue
+    }
+    let start = end
+    while (scan.charAt(start - 1) === '`') start -= 1
+    let segmentEnd = end
+    for (let index = end; index > start; index -= 1) {
+      if (!escaped.has(index)) continue
+      formed.add(segmentEnd - index + 1)
+      segmentEnd = index - 1
+    }
+    if (segmentEnd !== end || escaped.has(start)) formed.add(segmentEnd - start + 1)
+    else if (escapings[start] !== 'none' && formed.has(end - start + 1)) {
+      for (let index = start; index <= end; index += 1) escaped.add(index)
+      formed.add(1)
+    }
+    end = start - 1
   }
 }