[LayoutNG] Fieldset content with negative margin block start

Handle fragmentation for fieldset content with a negative margin block
start. The following was required to accomplish this:

1. Don't wait to layout fieldset content until there is available
space in the current fragment.
2. Break before fieldset content if necessary.

Bug: 875235
Change-Id: I0c95fe37694e2a0022ed9222db0a7af13cc958b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2099284
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750233}

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

@@ -173,24 +173,16 @@ scoped_refptr<const NGLayoutResult> NGFieldsetLayoutAlgorithm::Layout() {
   borders_with_legend.block_start = block_start_padding_edge;
   LogicalSize adjusted_padding_box_size =
       ShrinkAvailableSize(border_box_size, borders_with_legend);
-  if (content_break_token ||
+  if ((IsResumingLayout(content_break_token.get())) ||
       (!block_start_padding_edge_adjusted && IsResumingLayout(BreakToken()))) {
     borders_with_legend.block_start = LayoutUnit();
   }
   LayoutUnit intrinsic_block_size = borders_with_legend.BlockSum();
 
-  bool has_space_left = true;
-  if (ConstraintSpace().HasKnownFragmentainerBlockSize()) {
-    LayoutUnit space_left =
-        FragmentainerSpaceAtBfcStart(ConstraintSpace()) - intrinsic_block_size;
-    has_space_left = space_left > 0;
-  }
-
   // Proceed with normal fieldset children (excluding the rendered legend). They
   // all live inside an anonymous child box of the fieldset container.
   auto fieldset_content = Node().GetFieldsetContent();
-  if (has_space_left && fieldset_content &&
-      (content_break_token || !has_seen_all_children)) {
+  if (fieldset_content && (content_break_token || !has_seen_all_children)) {
     auto child_space = CreateConstraintSpaceForFieldsetContent(
         fieldset_content, adjusted_padding_box_size,
         borders_with_legend.block_start);
@@ -200,26 +192,34 @@ scoped_refptr<const NGLayoutResult> NGFieldsetLayoutAlgorithm::Layout() {
     // TODO(layout-dev): Handle abortions caused by block fragmentation.
     DCHECK_EQ(result->Status(), NGLayoutResult::kSuccess);
 
-    container_builder_.AddResult(*result, borders_with_legend.StartOffset());
+    NGBreakStatus break_status = NGBreakStatus::kContinue;
+    if (ConstraintSpace().HasBlockFragmentation()) {
+      LayoutUnit block_offset =
+          ConstraintSpace().FragmentainerOffsetAtBfc() + intrinsic_block_size;
+      break_status = BreakBeforeChildIfNeeded(
+          ConstraintSpace(), fieldset_content, *result.get(), block_offset,
+          /*has_container_separation*/ true, &container_builder_);
+    }
 
-    const auto& physical_fragment = result->PhysicalFragment();
-    intrinsic_block_size +=
-        NGFragment(writing_mode, physical_fragment).BlockSize();
-    container_builder_.SetHasSeenAllChildren();
+    if (break_status == NGBreakStatus::kContinue) {
+      container_builder_.AddResult(*result, borders_with_legend.StartOffset());
+      intrinsic_block_size +=
+          NGFragment(writing_mode, result->PhysicalFragment()).BlockSize();
+      container_builder_.SetHasSeenAllChildren();
+    }
   }
 
-  LayoutUnit consumed_block_size =
-      BreakToken() ? BreakToken()->ConsumedBlockSize() : LayoutUnit();
   if (!fieldset_content) {
     container_builder_.SetHasSeenAllChildren();
     // There was no anonymous child to provide the padding, so we have to add it
-    // ourselves. Subtract out the consumed block size to avoid over-calculating
-    // the block size when the fieldset's height is auto.
+    // ourselves.
     intrinsic_block_size += padding.BlockSum();
   }
 
   intrinsic_block_size = ClampIntrinsicBlockSize(
       ConstraintSpace(), Node(), adjusted_border_padding, intrinsic_block_size);
+  LayoutUnit consumed_block_size =
+      BreakToken() ? BreakToken()->ConsumedBlockSize() : LayoutUnit();
 
   // Recompute the block-axis size now that we know our content size.
   border_box_size.block_size =

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

@@ -514,7 +514,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, MinMax) {
 // line.
 TEST_F(NGFieldsetLayoutAlgorithmTest, NoFragmentation) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px; height: 100px;
@@ -541,7 +540,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, NoFragmentation) {
 // Tests that a fieldset will fragment if it reaches the fragmentation line.
 TEST_F(NGFieldsetLayoutAlgorithmTest, SimpleFragmentation) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         #fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px; height: 500px;
@@ -617,7 +615,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, FragmentationNoPadding) {
 // the fragmentation line.
 TEST_F(NGFieldsetLayoutAlgorithmTest, FieldsetContentFragmentationAutoHeight) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         #fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px;
@@ -680,7 +677,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, FieldsetContentFragmentationAutoHeight) {
 // reaches the fragmentation line.
 TEST_F(NGFieldsetLayoutAlgorithmTest, FieldsetContentFragmentation) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         #fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px; height: 100px;
@@ -743,7 +739,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, FieldsetContentFragmentation) {
 // the fragmentation line.
 TEST_F(NGFieldsetLayoutAlgorithmTest, LegendFragmentationAutoHeight) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         #fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px;
@@ -805,7 +800,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendFragmentationAutoHeight) {
 // encompass the legend.
 TEST_F(NGFieldsetLayoutAlgorithmTest, LegendFragmentation) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         #fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px; height: 100px;
@@ -866,7 +860,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendFragmentation) {
 // reaches the fragmentation line.
 TEST_F(NGFieldsetLayoutAlgorithmTest, LegendAndContentFragmentationAutoHeight) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         #fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px;
@@ -944,7 +937,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendAndContentFragmentationAutoHeight) {
 // reaches the fragmentation line.
 TEST_F(NGFieldsetLayoutAlgorithmTest, LegendAndContentFragmentation) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         #fieldset {
           border:3px solid; margin:0; padding:10px; width: 150px; height: 100px;
@@ -1021,7 +1013,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendAndContentFragmentation) {
 // Tests fragmentation when a legend's child content overflows.
 TEST_F(NGFieldsetLayoutAlgorithmTest, LegendFragmentationWithOverflow) {
   SetBodyInnerHTML(R"HTML(
-      <!DOCTYPE html>
       <style>
         fieldset, legend { margin:0; border:none; padding:0; }
       </style>
@@ -1070,5 +1061,62 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendFragmentationWithOverflow) {
   EXPECT_EQ(expectation, dump);
 }
 
+// Tests that fragmentation works as expected when the fieldset content has a
+// negative margin block start.
+TEST_F(NGFieldsetLayoutAlgorithmTest,
+       LegendAndContentFragmentationNegativeMargin) {
+  SetBodyInnerHTML(R"HTML(
+      <style>
+        #fieldset {
+          border:none; margin:0; padding:0px; width: 150px; height: 100px;
+        }
+        #legend {
+          padding:0px; margin:0; width: 50px; height: 100px;
+        }
+        #child {
+          margin-top: -20px; width: 100px; height: 40px;
+        }
+      </style>
+      <fieldset id="fieldset">
+        <legend id="legend"></legend>
+        <div id="child"></div>
+      </fieldset>
+  )HTML");
+
+  LayoutUnit kFragmentainerSpaceAvailable(100);
+
+  NGBlockNode node(ToLayoutBox(GetLayoutObjectByElementId("fieldset")));
+  NGConstraintSpace space = ConstructBlockLayoutTestConstraintSpace(
+      WritingMode::kHorizontalTb, TextDirection::kLtr,
+      LogicalSize(LayoutUnit(1000), kIndefiniteSize), false,
+      node.CreatesNewFormattingContext(), kFragmentainerSpaceAvailable);
+
+  scoped_refptr<const NGPhysicalBoxFragment> fragment =
+      NGBaseLayoutAlgorithmTest::RunFieldsetLayoutAlgorithm(node, space);
+  ASSERT_FALSE(fragment->BreakToken()->IsFinished());
+
+  String dump = DumpFragmentTree(fragment.get());
+  String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
+  offset:unplaced size:150x100
+    offset:0,0 size:50x100
+    offset:0,100 size:150x0
+      offset:0,-20 size:100x20
+)DUMP";
+  EXPECT_EQ(expectation, dump);
+
+  fragment = NGBaseLayoutAlgorithmTest::RunFieldsetLayoutAlgorithm(
+      node, space, fragment->BreakToken());
+  ASSERT_FALSE(fragment->BreakToken());
+
+  // TODO(almaher): The second node should not be 100px tall.
+  dump = DumpFragmentTree(fragment.get());
+  expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
+  offset:unplaced size:150x0
+    offset:0,0 size:150x100
+      offset:0,0 size:100x20
+)DUMP";
+  EXPECT_EQ(expectation, dump);
+}
+
 }  // anonymous namespace
 }  // namespace blink