Fix 8.5.17 visitor regression

lib/lazy-result.js

@@ -503,14 +503,24 @@ class LazyResult {
 
     if (visit.iterator !== 0) {
       let iterator = visit.iterator
+      // Advance past the child we just finished visiting. Like
+      // `Container#each`, the index is incremented only after a child has
+      // been fully processed, so a node inserted right after the current
+      // child is not skipped by the `existIndex < index` adjustment in
+      // `Container#insertAfter()` (which would fire exit events too early).
+      if (visit.descending) {
+        visit.descending = false
+        node.indexes[iterator] += 1
+      }
       let child
       while ((child = node.nodes[node.indexes[iterator]])) {
-        node.indexes[iterator] += 1
         if (!child[isClean]) {
           child[isClean] = true
+          visit.descending = true
           stack.push(toStack(child))
           return
         }
+        node.indexes[iterator] += 1
       }
       visit.iterator = 0
       delete node.indexes[iterator]
@@ -548,12 +558,22 @@ class LazyResult {
 
       if (visit.iterator !== 0) {
         let iterator = visit.iterator
+        // Advance past the child we just finished visiting. Like
+        // `Container#each`, the index is incremented only after a child has
+        // been fully processed. Incrementing before (as this loop used to)
+        // makes a node inserted right after the current child get skipped by
+        // the `existIndex < index` adjustment in `Container#insertAfter()`,
+        // which fires exit events before those new siblings are visited.
+        if (visit.descending) {
+          visit.descending = false
+          visitNode.indexes[iterator] += 1
+        }
         let child
         let descended = false
         while ((child = visitNode.nodes[visitNode.indexes[iterator]])) {
-          visitNode.indexes[iterator] += 1
           if (!child[isClean]) {
             child[isClean] = true
+            visit.descending = true
             stack.push({
               eventIndex: 0,
               events: getEvents(child),
@@ -563,6 +583,7 @@ class LazyResult {
             descended = true
             break
           }
+          visitNode.indexes[iterator] += 1
         }
         if (descended) continue
         visit.iterator = 0

test/visitor.test.ts

@@ -324,6 +324,50 @@ test('works with at-rule params', () => {
   is(css, '@media (mobile) {}')
 })
 
+function trackUnwrap(): [string[], Plugin] {
+  let order: string[] = []
+  let plugin: Plugin = {
+    postcssPlugin: 'unwrap-nested',
+    Rule(rule) {
+      order.push('Rule ' + rule.selector)
+      rule.each(child => {
+        if (child.type === 'rule') {
+          child.selector = rule.selector + ' ' + child.selector
+          rule.after(child)
+        }
+      })
+    },
+    RootExit() {
+      order.push('RootExit')
+    }
+  }
+  return [order, plugin]
+}
+
+test('visits nodes inserted after the current one before exit events', () => {
+  let [order, plugin] = trackUnwrap()
+  postcss([plugin]).process('a { b { c {} } }', { from: 'a.css' }).css
+  equal(order, [
+    'Rule a',
+    'Rule a b',
+    'Rule a b c',
+    'RootExit',
+    'Rule a',
+    'Rule a b',
+    'RootExit'
+  ])
+})
+
+test('visits inserted siblings equally in sync and async walks', async () => {
+  let [syncOrder, syncPlugin] = trackUnwrap()
+  postcss([syncPlugin]).process('a { b { c {} } }', { from: 'a.css' }).css
+
+  let [asyncOrder, asyncPlugin] = trackUnwrap()
+  await postcss([asyncPlugin]).process('a { b { c {} } }', { from: 'a.css' })
+
+  equal(asyncOrder, syncOrder)
+})
+
 test('wraps node to proxies', () => {
   let proxy: any
   let root: Root | undefined