5 files changed, 97 insertions, 5 deletions
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 1341f9633..85f53a820 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -6076,12 +6076,10 @@ nsCSSFrameConstructor::AddFrameConstructionItemsInternal(nsFrameConstructorState
 static void
 AddGenConPseudoToFrame(nsIFrame* aOwnerFrame, nsIContent* aContent)
 {
-  NS_ASSERTION(nsLayoutUtils::IsFirstContinuationOrIBSplitSibling(aOwnerFrame),
-               "property should only be set on first continuation/ib-sibling");
-
   // FIXME(emilio): Remove this property, and use the frame of the generated
   // content itself to tear the content down? It should be quite simpler.
 
+  aOwnerFrame = nsLayoutUtils::FirstContinuationOrIBSplitSibling(aOwnerFrame);
   nsIFrame::ContentArray* value =
     aOwnerFrame->GetProperty(nsIFrame::GenConProperty());
   if (!value) {
@@ -6343,7 +6341,9 @@ AdjustAppendParentForAfterContent(nsFrameManager* aFrameManager,
   // frames to find the first one that is either a ::after frame for an
   // ancestor of aChild or a frame that is for a node later in the
   // document than aChild and return that in aAfterFrame.
-  if (aParentFrame->GetProperty(nsIFrame::GenConProperty()) ||
+  nsIFrame* afterBeforeOwnerFrame =
+    nsLayoutUtils::FirstContinuationOrIBSplitSibling(aParentFrame);
+  if (afterBeforeOwnerFrame->GetProperty(nsIFrame::GenConProperty()) ||
       nsLayoutUtils::HasPseudoStyle(aContainer, aParentFrame->StyleContext(),
                                     CSSPseudoElementType::after,
                                     aParentFrame->PresContext()) ||
diff --git a/layout/generic/crashtests/1381134-2.html b/layout/generic/crashtests/1381134-2.html
new file mode 100644
index 000000000..d3ac73507
--- /dev/null
+++ b/layout/generic/crashtests/1381134-2.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="UTF-8">
+<script>
+addEventListener("DOMContentLoaded", () => {
+  [d1, d2] = document.getElementsByTagName("div");
+  [s1, s2] = document.getElementsByTagName("span")
+  d3 = document.createElement("div")
+  d4 = document.createElement("div")
+  d4.setAttribute("class", "grid")
+  d3.appendChild(d4)
+  d1.appendChild(document.createElement("span"))
+  setTimeout(() => {
+    d2.removeChild(s2)
+    setTimeout(() => {
+      d2.insertBefore(d3, s1)
+    }, 100)
+  }, 100)
+})
+</script>
+<style>
+.columns {
+  columns: 3;
+}
+.grid {
+  border:5px solid;
+  counter-reset: item;
+}
+.grid * { display:block; }
+span { display:contents; }
+span::before { content: counter(item) ":before"; }
+span::after { content: counter(item) ":after"; }
+</style>
+</head>
+<body>
+<div class=columns>
+<div class=grid>
+<c></c>
+<span><c></c></span>
+<span><c></c></span>
+</div>
+</div>
+</body>
+</html>
diff --git a/layout/generic/crashtests/1381134.html b/layout/generic/crashtests/1381134.html
new file mode 100644
index 000000000..a45fa04ec
--- /dev/null
+++ b/layout/generic/crashtests/1381134.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="UTF-8">
+<script>
+addEventListener("DOMContentLoaded", () => {
+  [d1, d2] = document.getElementsByTagName("div");
+  [s1, s2] = document.getElementsByTagName("span")
+  d3 = document.createElement("div")
+  d4 = document.createElement("div")
+  d4.setAttribute("class", "grid")
+  d3.appendChild(d4)
+  d1.appendChild(document.createElement("span"))
+  setTimeout(() => {
+    d2.removeChild(s2)
+    setTimeout(() => {
+      d2.insertBefore(d3, s1)
+    }, 100)
+  }, 100)
+})
+</script>
+<style>
+.columns {
+  columns: 3;
+}
+.grid {
+  display: grid;
+  border:5px solid;
+  counter-reset: item;
+}
+span { display:contents; }
+span::before { content: counter(item) ":before"; }
+span::after { content: counter(item) ":after"; }
+</style>
+</head>
+<body>
+<div class=columns>
+<div class=grid>
+<c></c>
+<span><c></c></span>
+<span><c></c></span>
+</div>
+</div>
+</body>
+</html>
diff --git a/layout/generic/crashtests/crashtests.list b/layout/generic/crashtests/crashtests.list
index a3c0d62c6..1a597e51c 100644
--- a/layout/generic/crashtests/crashtests.list
+++ b/layout/generic/crashtests/crashtests.list
@@ -642,3 +642,5 @@ load 1278461-1.html
 load 1278461-2.html
 load 1304441.html
 load 1316649.html
+load 1381134.html
+load 1381134-2.html
diff --git a/layout/reftests/css-grid/reftest.list b/layout/reftests/css-grid/reftest.list
index 7c5e6be51..52073b9bf 100644
--- a/layout/reftests/css-grid/reftest.list
+++ b/layout/reftests/css-grid/reftest.list
@@ -251,7 +251,7 @@ asserts(0-10) == grid-fragmentation-015.html grid-fragmentation-015-ref.html # b
 != grid-fragmentation-dyn1-021.html grid-fragmentation-021-ref.html # bug 1251799
 == grid-fragmentation-dyn2-021.html grid-fragmentation-021-ref.html
 == grid-fragmentation-dyn3-021.html grid-fragmentation-021-ref.html
-asserts(1-10) == grid-fragmentation-dyn4-021.html grid-fragmentation-021-ref.html # assertion related to bug 1251799 ?
+== grid-fragmentation-dyn4-021.html grid-fragmentation-021-ref.html
 == grid-fragmentation-dyn5-021.html grid-fragmentation-021-ref.html
 == grid-fragmentation-dyn2-022.html grid-fragmentation-007-ref.html
 == grid-fragmentation-dyn1-023.html grid-fragmentation-023-ref.html