Fieldset NG: Fix block-direction position of LEGEND

According to the legacy layout behavior, WebKit behavior, and Firefox
behavior, margins of a LEGEND should not affect block-direction
position of the LEGEND. The border-box of the LEGEND should be centered
vertically in the top border of the FIELDSET.

- We should compare the block-start border width of the FIELDSET and
  the block size of the LEGEND border-box.

- Even if the block size of LEGEND border-box is smaller than the
  block-start border of the FIELDSET, block-end margin of the LEGEND
  can affect the position of the FIELDSET content box.

This fixes legend-after-margin-with-before-border-horizontal-mode.html
and legend-small-after-margin-before-border-horizontal-mode.html, and
the major part of legend-block-margins-2.html.

Test changes:
 * NGFieldsetLayoutAlgorithmTest.LegendBreakBeforeAvoid
  The test assumed a break happened in the block-start margin of a
  LEGEND. We ignore the block-start margin, and the test broke inside
  the LEGEND. Add 'break-inside:avoid' to avoid breaking inside the
  LEGEND.

 * NGFieldsetLayoutAlgorithmTest.MarginTopPastEndOfFragmentainer
  Removed. The test made sure that if a break happens inside the
  block-start margin of a LEGEND, the reaiming block-start margin was
  ignored.  We ignore the block-start margin, so we can't test the
  behavior.

Bug: 875235
Change-Id: I78db12c42d5d1d5a928b7871838a5330f4b6943a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2331995
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#794889}

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

@@ -270,7 +270,7 @@ NGBreakStatus NGFieldsetLayoutAlgorithm::LayoutLegend(
 
   scoped_refptr<const NGLayoutResult> result;
   scoped_refptr<const NGLayoutResult> previous_result;
-  LayoutUnit block_offset = legend_margins.block_start;
+  LayoutUnit block_offset;
   do {
     auto legend_space = CreateConstraintSpaceForLegend(
         legend, ChildAvailableSize(), percentage_size, block_offset);
@@ -296,19 +296,20 @@ NGBreakStatus NGFieldsetLayoutAlgorithm::LayoutLegend(
 
     // We have already adjusted the legend block offset, no need to adjust
     // again.
-    if (block_offset != legend_margins.block_start) {
+    if (block_offset != LayoutUnit()) {
       // If adjusting the block_offset caused the legend to break, revert back
       // to the previous result.
       if (legend_broke_) {
         result = std::move(previous_result);
-        block_offset = legend_margins.block_start;
+        block_offset = LayoutUnit();
       }
       break;
     }
 
-    LayoutUnit legend_margin_box_block_size =
-        legend_margins.block_start +
+    LayoutUnit legend_border_box_block_size =
         NGFragment(writing_mode_, physical_fragment).BlockSize();
+    LayoutUnit legend_margin_box_block_size =
+        legend_margins.block_start + legend_border_box_block_size;
 
     LayoutUnit block_end_margin = legend_margins.block_end;
     if (ConstraintSpace().HasKnownFragmentainerBlockSize()) {
@@ -317,18 +318,16 @@ NGBreakStatus NGFieldsetLayoutAlgorithm::LayoutLegend(
     }
     legend_margin_box_block_size += block_end_margin;
 
-    LayoutUnit space_left = borders_.block_start - legend_margin_box_block_size;
-
+    LayoutUnit space_left = borders_.block_start - legend_border_box_block_size;
     if (space_left > LayoutUnit()) {
       // Don't adjust the block-start offset of the legend if the legend broke.
       if (legend_break_token || legend_broke_)
         break;
 
-      // If the border is the larger one, though, it will stay put at the
-      // border-box block-start edge of the fieldset. Then it's the legend
-      // that needs to be pushed. We'll center the margin box in this case, to
-      // make sure that both margins remain within the area occupied by the
-      // border also after adjustment.
+      // https://html.spec.whatwg.org/C/#the-fieldset-and-legend-elements
+      // * The element is expected to be positioned in the block-flow direction
+      //   such that its border box is centered over the border on the
+      //   block-start side of the fieldset element.
       block_offset += space_left / 2;
       if (ConstraintSpace().HasBlockFragmentation()) {
         // Save the previous result in case adjusting the block_offset causes
@@ -336,23 +335,28 @@ NGBreakStatus NGFieldsetLayoutAlgorithm::LayoutLegend(
         previous_result = std::move(result);
         continue;
       }
-    } else {
-      // If the border is smaller, intrinsic_block_size_ should now be based on
-      // the size of the legend instead of the border.
-      intrinsic_block_size_ = legend_margin_box_block_size;
+    }
+    // If the border is smaller than the block end offset of the legend margin
+    // box, intrinsic_block_size_ should now be based on the the block end
+    // offset of the legend margin box instead of the border.
+    LayoutUnit legend_margin_end_offset = block_offset +
+                                          legend_margin_box_block_size -
+                                          legend_margins.block_start;
+    if (legend_margin_end_offset > borders_.block_start) {
+      intrinsic_block_size_ = legend_margin_end_offset;
 
       is_legend_past_border_ = true;
 
       // Don't adjust the block-start offset of the fragment border if it broke.
       if (BreakToken() || (ConstraintSpace().HasKnownFragmentainerBlockSize() &&
-                           legend_margin_box_block_size >
+                           legend_margin_end_offset >
                                ConstraintSpace().FragmentainerBlockSize()))
         break;
       // If the legend is larger than the width of the fieldset block-start
       // border, the actual padding edge of the fieldset will be moved
       // accordingly. This will be the block-start offset for the fieldset
       // contents anonymous box.
-      borders_.block_start = legend_margin_box_block_size;
+      borders_.block_start = legend_margin_end_offset;
     }
     break;
   } while (true);

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

@@ -156,7 +156,7 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, SmallLegendLargeBorder) {
   String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
   offset:unplaced size:1000x200
     offset:0,0 size:200x200
-      offset:50,10 size:10x10
+      offset:50,15 size:10x10
       offset:40,40 size:120x120
         offset:10,10 size:100x100
 )DUMP";
@@ -180,10 +180,10 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendOrthogonalWritingMode) {
 
   String dump = DumpFragmentTree(GetElementById("container"));
   String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
-  offset:unplaced size:1000x203
-    offset:0,0 size:126x203
-      offset:43,10 size:10x50
-      offset:3,80 size:120x120
+  offset:unplaced size:1000x193
+    offset:0,0 size:126x193
+      offset:43,0 size:10x50
+      offset:3,70 size:120x120
         offset:10,10 size:100x100
 )DUMP";
   EXPECT_EQ(expectation, dump);
@@ -207,9 +207,9 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, VerticalLr) {
   String dump = DumpFragmentTree(GetElementById("container"));
   String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
   offset:unplaced size:1000x126
-    offset:0,0 size:178x126
-      offset:30,23 size:10x50
-      offset:55,3 size:120x120
+    offset:0,0 size:148x126
+      offset:0,23 size:10x50
+      offset:25,3 size:120x120
         offset:10,10 size:100x100
 )DUMP";
   EXPECT_EQ(expectation, dump);
@@ -233,7 +233,7 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, VerticalRl) {
   String dump = DumpFragmentTree(GetElementById("container"));
   String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
   offset:unplaced size:1000x126
-    offset:0,0 size:178x126
+    offset:0,0 size:163x126
       offset:153,23 size:10x50
       offset:3,3 size:120x120
         offset:10,10 size:100x100
@@ -1530,7 +1530,7 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendBreakBeforeAvoid) {
     <div id="container">
       <div style="width:20px; height:90px;"></div>
       <fieldset id="fieldset">
-        <legend id="legend" style="break-before:avoid;"></legend>
+        <legend id="legend" style="break-before:avoid; break-inside:avoid"></legend>
       </fieldset>
     </div>
   )HTML");
@@ -1560,10 +1560,10 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendBreakBeforeAvoid) {
 
   dump = DumpFragmentTree(fragment.get());
   expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
-  offset:unplaced size:1000x55
-    offset:0,0 size:120x55
-      offset:20,10 size:10x25
-      offset:10,45 size:100x0
+  offset:unplaced size:1000x45
+    offset:0,0 size:120x45
+      offset:20,0 size:10x25
+      offset:10,35 size:100x0
 )DUMP";
   EXPECT_EQ(expectation, dump);
 }
@@ -1677,58 +1677,6 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, LegendBreakAfterAvoid) {
   EXPECT_EQ(expectation, dump);
 }
 
-TEST_F(NGFieldsetLayoutAlgorithmTest, MarginTopPastEndOfFragmentainer) {
-  // A block whose border box would start past the end of the current
-  // fragmentainer should start exactly at the start of the next fragmentainer,
-  // discarding what's left of the margin.
-  // https://www.w3.org/TR/css-break-3/#break-margins
-  SetBodyInnerHTML(R"HTML(
-    <style>
-      #fieldset {
-        border:none; margin:0; padding:0px; width: 100px; height: 100px;
-      }
-      #legend {
-        padding:0px; margin:0px;
-      }
-    </style>
-     <fieldset id="fieldset">
-      <legend id="legend" style="margin-top:60px; width:10px; height:20px;"></legend>
-      <div style="width:20px; height:20px;"></div>
-    </fieldset>
-  )HTML");
-
-  LayoutUnit kFragmentainerSpaceAvailable(50);
-
-  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_TRUE(fragment->BreakToken());
-
-  String dump = DumpFragmentTree(fragment.get());
-  String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
-  offset:unplaced size:100x50
-)DUMP";
-  EXPECT_EQ(expectation, dump);
-
-  fragment = NGBaseLayoutAlgorithmTest::RunFieldsetLayoutAlgorithm(
-      node, space, fragment->BreakToken());
-  ASSERT_FALSE(fragment->BreakToken());
-
-  dump = DumpFragmentTree(fragment.get());
-  expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
-  offset:unplaced size:100x50
-    offset:0,0 size:10x20
-    offset:0,20 size:100x30
-      offset:0,0 size:20x20
-)DUMP";
-  EXPECT_EQ(expectation, dump);
-}
-
 TEST_F(NGFieldsetLayoutAlgorithmTest, MarginBottomPastEndOfFragmentainer) {
   // A block whose border box would start past the end of the current
   // fragmentainer should start exactly at the start of the next fragmentainer,
@@ -1982,7 +1930,7 @@ TEST_F(NGFieldsetLayoutAlgorithmTest, SmallerLegendLargeBorderWithBreak) {
   String dump = DumpFragmentTree(fragment.get());
   String expectation = R"DUMP(.:: LayoutNG Physical Fragment Tree ::.
   offset:unplaced size:220x60
-    offset:60,16 size:10x5
+    offset:60,27.5 size:10x5
 )DUMP";
   EXPECT_EQ(expectation, dump);
 

third_party/blink/web_tests/TestExpectations

@@ -1308,10 +1308,6 @@ virtual/layout_ng_fragment_traversal/external/wpt/css/CSS2/text/* [ Skip ]
 # Fieldset in NG
 #
 
-### virtual/layout_ng_fieldset/fast/forms/fieldset/
-crbug.com/875235 virtual/layout_ng_fieldset/fast/forms/fieldset/legend-small-after-margin-before-border-horizontal-mode.html [ Failure ]
-crbug.com/875235 virtual/layout_ng_fieldset/fast/forms/fieldset/legend-after-margin-with-before-border-horizontal-mode.html [ Failure ]
-
 ## Fieldseet in NG - Passing reference tests
 crbug.com/875235 virtual/layout_ng_fieldset/external/wpt/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/fieldset-vertical.html [ Pass ]