import from UXP: Bug 1419762 - Return the inline continuation of an IB split when appending (fc8fcfc0)

layout/base/crashtests/1419762.html

@@ -0,0 +1,15 @@
+<style id='style_1'>
+  :first-child { display: table-column-group; }
+</style>
+<script>
+  try { o1 = document.createElement('isindex') } catch(e) { }
+  try { o2 = document.createElement('input') } catch(e) { }
+  try { o3 = document.createElement('optgroup') } catch(e) { }
+  try { o4 = document.createElement('col') } catch(e) { }
+  try { document.documentElement.appendChild(o1) } catch(e) { }
+  try { o1.appendChild(o2) } catch(e) { }
+  try { o1.appendChild(o3) } catch(e) { }
+  try { document.documentElement.offsetTop; } catch (e) { }
+  try { document.documentElement.appendChild(o4) } catch(e) { }
+  try { document.styleSheets[0].insertRule('optgroup::first-line { list-style-type: japanese-formal; }', 0); } catch(e) { }
+</script>

layout/base/crashtests/crashtests.list

@@ -485,3 +485,4 @@ load 1308848-2.html
 asserts(0-1) load 1343606.html # bug 1343948
 load 1404789-1.html
 load 1404789-2.html
+load 1419762.html

layout/base/nsCSSFrameConstructor.cpp

@@ -6796,10 +6796,11 @@ nsCSSFrameConstructor::GetInsertionPrevSibling(InsertionPoint* aInsertion,
       *aIsAppend = true;
       if (IsFramePartOfIBSplit(aInsertion->mParentFrame)) {
         // Since we're appending, we'll walk to the last anonymous frame
-        // that was created for the broken inline frame.  But don't walk
-        // to the trailing inline if it's empty; stop at the block.
+        // that was created for the broken inline frame. We can walk to the
+        // trailing inline, since we know this is a real append, and not an
+        // insert (that would've been handled by `FindNextSibling`).
         aInsertion->mParentFrame =
-          GetLastIBSplitSibling(aInsertion->mParentFrame, false);
+          GetLastIBSplitSibling(aInsertion->mParentFrame, true);
       }
       // Get continuation that parents the last child.
       aInsertion->mParentFrame =