[LayoutNG] Delay layout of OOF elements inside a nested fragmentainer

Currently, we wait to layout fragmented OOF elements until they've
reached the fragmentation context root. In the case of nested
fragmentation, we need to wait until that fragmentation context
root has finished layout to ensure that the containing block has
also finished layout.

This change delays layout of fragmented OOF elements until we've
reached a fragmentation context root that is not itself fragmented.
This results in OOF elements being added as children to the outermost
fragmentainer that it is found in rather than the innermost.

We will need to bubble up some extra information about the innermost
fragmentainer in order to properly position the OOF elements. This
will be done in a follow-up CL.

Bug: 1079031
Change-Id: Ia2d5ed3af2a99f5c091d1cd9c399cc34bc580038
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462048
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815698}

third_party/blink/renderer/core/layout/ng/ng_out_of_flow_layout_part.cc

@@ -129,6 +129,7 @@ NGOutOfFlowLayoutPart::NGOutOfFlowLayoutPart(
 
 void NGOutOfFlowLayoutPart::Run(const LayoutBox* only_layout) {
   if (container_builder_->IsBlockFragmentationContextRoot() &&
+      !container_space_.HasBlockFragmentation() &&
       container_builder_->HasOutOfFlowFragmentainerDescendants()) {
     Vector<NGLogicalOutOfFlowPositionedNode> fragmentainer_descendants;
     container_builder_->SwapOutOfFlowFragmentainerDescendants(

third_party/blink/renderer/core/layout/ng/ng_out_of_flow_layout_part_test.cc

@@ -1247,5 +1247,65 @@ TEST_F(NGOutOfFlowLayoutPartTest,
   EXPECT_EQ(expectation, dump);
 }
 
+// Fragmented OOF element inside a nested multi-column.
+TEST_F(NGOutOfFlowLayoutPartTest, AbsposNestedFragmentation) {
+  SetBodyInnerHTML(
+      R"HTML(
+      <style>
+        .multicol {
+          columns:2; column-fill:auto; column-gap:0px;
+        }
+        .rel {
+          position: relative; width:55px;
+        }
+        .abs {
+          position:absolute; top:0px; bottom:0px; width:5px;
+        }
+      </style>
+      <div id="container">
+        <div class="multicol" id="outer" style="height:100px;">
+          <div style="height:40px; width:40px;"></div>
+          <div class="multicol" id="inner">
+            <div class="rel">
+              <div class="abs"></div>
+              <div style="height:250px; width:25px;"></div>
+            </div>
+          </div>
+        </div>
+      </div>
+      )HTML");
+  String dump = DumpFragmentTree(GetElementById("container"));
+
+  // TODO(almaher): There should be two abspos fragments with height 60 in the
+  // first outer column, and two with height 100/30 in the second outer column.
+  // There should not be a third outer column.
+  String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
+  offset:unplaced size:1000x100
+    offset:0,0 size:1000x100
+      offset:0,0 size:500x100
+        offset:0,0 size:40x40
+        offset:0,40 size:500x60
+          offset:0,0 size:250x60
+            offset:0,0 size:55x60
+              offset:0,0 size:25x60
+          offset:250,0 size:250x60
+            offset:0,0 size:55x60
+              offset:0,0 size:25x60
+        offset:0,40 size:5x60
+      offset:500,0 size:500x100
+        offset:0,0 size:500x100
+          offset:0,0 size:250x100
+            offset:0,0 size:55x100
+              offset:0,0 size:25x100
+          offset:250,0 size:250x100
+            offset:0,0 size:55x30
+              offset:0,0 size:25x30
+        offset:0,0 size:5x100
+      offset:1000,0 size:500x100
+        offset:0,0 size:5x90
+)DUMP";
+  EXPECT_EQ(expectation, dump);
+}
+
 }  // namespace
 }  // namespace blink