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fix: prevent markdown fence balancer from corrupting sequential code blocks #26785

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Merged
dsyme merged 7 commits into from
Apr 17, 2026
Merged

fix: prevent markdown fence balancer from corrupting sequential code blocks #26785

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ab97698
fix: prevent markdown fence balancer from corrupting sequential code …
dsyme Apr 16, 2026
72b6bd7
fix: resolve TS2345 error in copilot_driver.cjs appendFileSync type
dsyme Apr 17, 2026
6e745c9
Merge branch 'main' into fix/markdown-fence-balancer-nesting-v2
dsyme Apr 17, 2026
1369515
refactor: remove dead nesting detection code, simplify to pure greedy…
dsyme Apr 17, 2026
8e59b44
fix: align isBalanced()/countCodeRegions() closer rule with CommonMark
dsyme Apr 17, 2026
ba3a0d0
Merge branch 'main' into fix/markdown-fence-balancer-nesting-v2
dsyme Apr 17, 2026
2e82516
test: update WASM golden files for agent folder ordering changes
dsyme Apr 17, 2026
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117 changes: 23 additions & 94 deletions actions/setup/js/markdown_code_region_balancer.cjs
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Original file line number Diff line number Diff line change
Expand Up @@ -119,14 +119,13 @@ function balanceCodeRegions(markdown) {
}
}

// Third pass: Match fences, detecting and fixing nested patterns
// Third pass: Match fences using greedy matching (CommonMark rules)
// Strategy:
// 1. Process fences in order
// 2. For each opener, find potential closers
// 3. If first closer has intermediate opener, defer this opener
// 4. Otherwise, pair with first direct closer (greedy matching)
// 5. Make a second pass for deferred openers
const fenceLengthAdjustments = new Map(); // lineIndex -> new length
const processed = new Set();
const deferred = new Set(); // Fences to process in second pass
const unclosedFences = [];
Expand Down Expand Up @@ -217,80 +216,20 @@ function balanceCodeRegions(markdown) {
processed.delete(i); // Unmark so it can be processed in second pass
i++;
} else {
// No opener before the first closer, so it's a direct match
// Check if there are MORE closers without intermediate openers
const directClosers = potentialClosers.filter(c => !c.hasOpenerBetween);

if (directClosers.length > 1) {
// Multiple bare closers without intermediate openers
// Count openers between our opener and the last direct closer to determine if this is true nesting
const lastDirectCloser = directClosers[directClosers.length - 1];
let openerCount = 0;
for (let k = i + 1; k < lastDirectCloser.index; k++) {
if (processed.has(k)) continue;
const intermediateFence = fences[k];
if (intermediateFence.language !== "" && intermediateFence.indent.length === openIndentLength) {
openerCount++;
}
}

// True nesting: more closers than openers (e.g., 1 opener, 3 closers)
// Nested blocks: closers = openers + 1 (e.g., 2 openers [including us], 2 closers)
const closerCount = directClosers.length;
const isTrueNesting = closerCount > openerCount + 1;

if (isTrueNesting) {
// TRUE nesting - use the LAST closer and escape middle ones
const closerIndex = lastDirectCloser.index;
processed.add(closerIndex);

pairedBlocks.push({
start: fences[i].lineIndex,
end: fences[closerIndex].lineIndex,
openIndex: i,
closeIndex: closerIndex,
});

// Increase fence length so middle closers can no longer close
const maxLength = Math.max(...directClosers.map(c => c.length), openFence.length);
const newLength = maxLength + 1;
fenceLengthAdjustments.set(fences[i].lineIndex, newLength);
fenceLengthAdjustments.set(fences[closerIndex].lineIndex, newLength);

// Mark middle closers as processed (they're now treated as content)
for (let k = 0; k < directClosers.length - 1; k++) {
processed.add(directClosers[k].index);
}

i = closerIndex + 1;
} else {
// Nested blocks - use the FIRST direct closer (greedy matching)
const closerIndex = directClosers[0].index;
processed.add(closerIndex);

pairedBlocks.push({
start: fences[i].lineIndex,
end: fences[closerIndex].lineIndex,
openIndex: i,
closeIndex: closerIndex,
});

i = closerIndex + 1;
}
} else {
// Only one direct closer, use it (normal case)
const closerIndex = firstCloser.index;
processed.add(closerIndex);

pairedBlocks.push({
start: fences[i].lineIndex,
end: fences[closerIndex].lineIndex,
openIndex: i,
closeIndex: closerIndex,
});

i = closerIndex + 1;
}
// No opener before the first closer — greedy match with it.
// Per CommonMark, the first bare fence that can close our opener does close it.
// Any subsequent bare fences start new blocks (sequential, not nested).
const closerIndex = firstCloser.index;
processed.add(closerIndex);

pairedBlocks.push({
start: fences[i].lineIndex,
end: fences[closerIndex].lineIndex,
openIndex: i,
closeIndex: closerIndex,
});

i = closerIndex + 1;
}
} else {
// No closer found - check if this fence is inside a paired block
Expand Down Expand Up @@ -347,23 +286,9 @@ function balanceCodeRegions(markdown) {
}
}

// Fifth pass: build result with adjusted fence lengths
// Fifth pass: build result (copy lines, then close any unclosed fences)
for (let i = 0; i < lines.length; i++) {
if (fenceLengthAdjustments.has(i)) {
const newLength = fenceLengthAdjustments.get(i);
const fenceMatch = lines[i].match(/^(\s*)(`{3,}|~{3,})([^`~\s]*)?(.*)$/);
if (fenceMatch) {
const indent = fenceMatch[1];
const char = fenceMatch[2][0];
const language = fenceMatch[3] || "";
const trailing = fenceMatch[4] || "";
result.push(`${indent}${char.repeat(newLength)}${language}${trailing}`);
} else {
result.push(lines[i]);
}
} else {
result.push(lines[i]);
}
result.push(lines[i]);
}

// Fifth pass: close any unclosed fences
Expand Down Expand Up @@ -428,7 +353,9 @@ function isBalanced(markdown) {
length: fenceLength,
};
} else {
const canClose = openingFence !== null && fenceChar === openingFence.char && fenceLength >= openingFence.length;
// Per CommonMark, a closing fence must be bare (no info string/language)
const language = fenceMatch[3] || "";
const canClose = openingFence !== null && fenceChar === openingFence.char && fenceLength >= openingFence.length && language === "";

if (canClose) {
inCodeBlock = false;
Expand Down Expand Up @@ -478,7 +405,9 @@ function countCodeRegions(markdown) {
length: fenceLength,
};
} else {
const canClose = openingFence !== null && fenceChar === openingFence.char && fenceLength >= openingFence.length;
// Per CommonMark, a closing fence must be bare (no info string/language)
const language = fenceMatch[3] || "";
const canClose = openingFence !== null && fenceChar === openingFence.char && fenceLength >= openingFence.length && language === "";

if (canClose) {
inCodeBlock = false;
Expand Down
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