Forced colors mode backplate (non-LayoutNG)

This change adds logic for painting a readability backplate behind
text in forced colors mode for non-LayoutNG. Backplates are split by
paragraph, as defined by two consecutive line breaks. A backplate is
not drawn if forced-color-adjust is none.

Tests are added for this in the LayoutNG change:
https://chromium-review.googlesource.com/c/chromium/src/+/1749299

Bug: 970285
Change-Id: I51aec229d905de5fe7b233792c41555037c5b172
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713333
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#691937}

third_party/blink/renderer/core/paint/block_painter.cc

@@ -270,8 +270,11 @@ void BlockPainter::PaintBlockFlowContents(const PaintInfo& paint_info,
   } else if (ShouldPaintDescendantOutlines(paint_info.phase)) {
     ObjectPainter(layout_block_).PaintInlineChildrenOutlines(paint_info);
   } else {
-    LineBoxListPainter(To<LayoutBlockFlow>(layout_block_).LineBoxes())
-        .Paint(layout_block_, paint_info, paint_offset);
+    LineBoxListPainter painter(To<LayoutBlockFlow>(layout_block_).LineBoxes());
+    // Draw a backplate behind all text if forced colors mode is enabled.
+    if (layout_block_.GetFrame()->GetDocument()->InForcedColorsMode())
+      painter.PaintBackplate(layout_block_, paint_info, paint_offset);
+    painter.Paint(layout_block_, paint_info, paint_offset);
   }
 
   // If we don't have any floats to paint, or we're in the wrong paint phase,

third_party/blink/renderer/core/paint/line_box_list_painter.cc

@@ -5,17 +5,77 @@
 #include "third_party/blink/renderer/core/paint/line_box_list_painter.h"
 
 #include "third_party/blink/renderer/core/layout/api/line_layout_box_model.h"
+#include "third_party/blink/renderer/core/layout/geometry/physical_rect.h"
 #include "third_party/blink/renderer/core/layout/layout_box_model_object.h"
 #include "third_party/blink/renderer/core/layout/line/inline_flow_box.h"
+#include "third_party/blink/renderer/core/layout/line/inline_text_box.h"
 #include "third_party/blink/renderer/core/layout/line/line_box_list.h"
 #include "third_party/blink/renderer/core/layout/line/root_inline_box.h"
 #include "third_party/blink/renderer/core/paint/object_painter.h"
 #include "third_party/blink/renderer/core/paint/paint_info.h"
 #include "third_party/blink/renderer/core/paint/paint_timing_detector.h"
-#include "third_party/blink/renderer/platform/graphics/paint/paint_controller.h"
+#include "third_party/blink/renderer/platform/graphics/color.h"
+#include "third_party/blink/renderer/platform/graphics/paint/drawing_recorder.h"
 
 namespace blink {
 
+namespace {
+
+// Recursively build up backplates behind inline text boxes, each split at the
+// paragraph level. Store the results in paragraph_backplates.
+void BuildBackplate(const InlineFlowBox* box,
+                    const PhysicalOffset& paint_offset,
+                    PhysicalRect* current_backplate,
+                    int* consecutive_line_breaks,
+                    Vector<PhysicalRect>* paragraph_backplates) {
+  DCHECK(current_backplate && consecutive_line_breaks && paragraph_backplates);
+
+  // The number of consecutive forced breaks that split the backplate by
+  // paragraph.
+  static constexpr int kMaxConsecutiveLineBreaks = 2;
+
+  // Build up and paint backplates of all child inline text boxes. We are not
+  // able to simply use the linebox rect to compute the backplate because the
+  // backplate should only be painted for inline text and not for atomic
+  // inlines.
+  for (InlineBox* child = box->FirstChild(); child;
+       child = child->NextOnLine()) {
+    LineLayoutItem layout_item = child->GetLineLayoutItem();
+    if (layout_item.IsText() || layout_item.IsListMarker()) {
+      if (layout_item.IsText()) {
+        String child_text =
+            ToInlineTextBox(child)->GetLineLayoutItem().GetText();
+        if (ToInlineTextBox(child)->IsLineBreak() ||
+            child_text.StartsWith('\n'))
+          (*consecutive_line_breaks)++;
+      }
+      if (*consecutive_line_breaks >= kMaxConsecutiveLineBreaks) {
+        // This is a paragraph point.
+        paragraph_backplates->push_back(*current_backplate);
+        *current_backplate = PhysicalRect();
+        *consecutive_line_breaks = 0;
+      }
+
+      PhysicalOffset box_origin(PhysicalOffset(child->Location()) +
+                                paint_offset);
+      PhysicalRect box_rect(box_origin, PhysicalSize(child->LogicalWidth(),
+                                                     child->LogicalHeight()));
+      if (*consecutive_line_breaks > 0 && !box_rect.IsEmpty()) {
+        // Text was reached, so reset consecutive_line_breaks.
+        *consecutive_line_breaks = 0;
+      }
+      current_backplate->Unite(box_rect);
+    } else if (child->IsInlineFlowBox()) {
+      // If an inline flow box was reached, continue to recursively build up the
+      // backplate.
+      BuildBackplate(ToInlineFlowBox(child), paint_offset, current_backplate,
+                     consecutive_line_breaks, paragraph_backplates);
+    }
+  }
+}
+
+}  // anonymous namespace
+
 static void AddPDFURLRectsForInlineChildrenRecursively(
     const LayoutObject& layout_object,
     const PaintInfo& paint_info,
@@ -31,7 +91,7 @@ static void AddPDFURLRectsForInlineChildrenRecursively(
   }
 }
 
-void LineBoxListPainter::Paint(const LayoutBoxModelObject& layout_object,
+bool LineBoxListPainter::ShouldPaint(const LayoutBoxModelObject& layout_object,
                                      const PaintInfo& paint_info,
                                      const PhysicalOffset& paint_offset) const {
   DCHECK(!ShouldPaintSelfOutline(paint_info.phase) &&
@@ -42,7 +102,7 @@ void LineBoxListPainter::Paint(const LayoutBoxModelObject& layout_object,
       paint_info.phase != PaintPhase::kSelection &&
       paint_info.phase != PaintPhase::kTextClip &&
       paint_info.phase != PaintPhase::kMask)
-    return;
+    return false;
 
   // The only way an inline could paint like this is if it has a layer.
   DCHECK(layout_object.IsLayoutBlock() ||
@@ -54,11 +114,20 @@ void LineBoxListPainter::Paint(const LayoutBoxModelObject& layout_object,
 
   // If we have no lines then we have no work to do.
   if (!line_box_list_.First())
-    return;
+    return false;
 
   if (!line_box_list_.AnyLineIntersectsRect(
           LineLayoutBoxModel(const_cast<LayoutBoxModelObject*>(&layout_object)),
           paint_info.GetCullRect(), paint_offset))
+    return false;
+
+  return true;
+}
+
+void LineBoxListPainter::Paint(const LayoutBoxModelObject& layout_object,
+                               const PaintInfo& paint_info,
+                               const PhysicalOffset& paint_offset) const {
+  if (!ShouldPaint(layout_object, paint_info, paint_offset))
     return;
 
   ScopedPaintTimingDetectorBlockPaintHook
@@ -88,4 +157,45 @@ void LineBoxListPainter::Paint(const LayoutBoxModelObject& layout_object,
   }
 }
 
+void LineBoxListPainter::PaintBackplate(
+    const LayoutBoxModelObject& layout_object,
+    const PaintInfo& paint_info,
+    const PhysicalOffset& paint_offset) const {
+  if (!ShouldPaint(layout_object, paint_info, paint_offset))
+    return;
+
+  // Only paint backplates behind text when forced-color-adjust is auto.
+  const ComputedStyle& style =
+      line_box_list_.First()->GetLineLayoutItem().StyleRef();
+  if (style.ForcedColorAdjust() == EForcedColorAdjust::kNone)
+    return;
+
+  if (DrawingRecorder::UseCachedDrawingIfPossible(
+          paint_info.context, layout_object,
+          DisplayItem::kForcedColorsModeBackplate))
+    return;
+
+  DrawingRecorder recorder(paint_info.context, layout_object,
+                           DisplayItem::kForcedColorsModeBackplate);
+  Color backplate_color = style.ForcedBackplateColor();
+  const auto& backplates = GetBackplates(paint_offset);
+  for (const auto backplate : backplates)
+    paint_info.context.FillRect(FloatRect(backplate), backplate_color);
+}
+
+Vector<PhysicalRect> LineBoxListPainter::GetBackplates(
+    const PhysicalOffset& paint_offset) const {
+  Vector<PhysicalRect> paragraph_backplates;
+  PhysicalRect current_backplate;
+  int consecutive_line_breaks = 0;
+  for (const InlineFlowBox* line : line_box_list_) {
+    // Recursively build up and paint backplates for line boxes containing text.
+    BuildBackplate(line, paint_offset, &current_backplate,
+                   &consecutive_line_breaks, &paragraph_backplates);
+  }
+  if (!current_backplate.IsEmpty())
+    paragraph_backplates.push_back(current_backplate);
+  return paragraph_backplates;
+}
+
 }  // namespace blink

third_party/blink/renderer/core/paint/line_box_list_painter.h

@@ -7,6 +7,7 @@
 
 #include "third_party/blink/renderer/core/style/computed_style_constants.h"
 #include "third_party/blink/renderer/platform/wtf/allocator/allocator.h"
+#include "third_party/blink/renderer/platform/wtf/vector.h"
 
 namespace blink {
 
@@ -14,6 +15,7 @@ class LayoutBoxModelObject;
 class LineBoxList;
 struct PaintInfo;
 struct PhysicalOffset;
+struct PhysicalRect;
 
 class LineBoxListPainter {
   STACK_ALLOCATED();
@@ -25,9 +27,20 @@ class LineBoxListPainter {
   void Paint(const LayoutBoxModelObject&,
              const PaintInfo&,
              const PhysicalOffset& paint_offset) const;
+  void PaintBackplate(const LayoutBoxModelObject& layout_object,
+                      const PaintInfo&,
+                      const PhysicalOffset& paint_offset) const;
 
  private:
   const LineBoxList& line_box_list_;
+
+  bool ShouldPaint(const LayoutBoxModelObject& layout_object,
+                   const PaintInfo& paint_info,
+                   const PhysicalOffset& paint_offset) const;
+
+  // Returns a vector of backplates that surround the paragraphs of text within
+  // line_box_list_.
+  Vector<PhysicalRect> GetBackplates(const PhysicalOffset& paint_offset) const;
 };
 
 }  // namespace blink

third_party/blink/web_tests/FlagExpectations/disable-blink-features=LayoutNG

@@ -304,9 +304,6 @@ crbug.com/591099 fast/css/forced-colors-mode/forced-colors-mode-15.html [ Failur
 crbug.com/591099 virtual/forced-colors/fast/css/forced-colors-mode/forced-colors-mode-14.html [ Failure ]
 crbug.com/591099 virtual/forced-colors/fast/css/forced-colors-mode/forced-colors-mode-15.html [ Failure ]
 
-# Backplate tests should not be run when LayoutNG is disabled until it is implemented there.
-crbug.com/970285 virtual/forced-colors/fast/css/forced-colors-mode/backplate [ Skip ]
-
 ### http/tests/credentialmanager/
 crbug.com/874695 http/tests/credentialmanager/credentialscontainer-create-with-virtual-authenticator.html [ Failure ]