Bug 1798810 - Update calculation of baseline for multicolumn containers. r=layout-reviewers,TYLin

First/Last Baseline should be from highest/lowest baseline in all columns and
spanners.

Differential Revision: https://phabricator.services.mozilla.com/D171261

layout/base/nsLayoutUtils.cpp

@@ -5838,8 +5838,7 @@ bool nsLayoutUtils::GetFirstLinePosition(WritingMode aWM,
       return false;
     }
 
-    if (fType == LayoutFrameType::FieldSet ||
-        fType == LayoutFrameType::ColumnSet) {
+    if (fType == LayoutFrameType::FieldSet) {
       LinePosition kidPosition;
       nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
       // If aFrame is fieldset, kid might be a legend frame here, but that's ok.
@@ -5851,6 +5850,29 @@ bool nsLayoutUtils::GetFirstLinePosition(WritingMode aWM,
       return false;
     }
 
+    if (fType == LayoutFrameType::ColumnSet) {
+      // Note(dshin): This is basically the same as
+      // `nsColumnSetFrame::GetNaturalBaselineBOffset`, but with line start and
+      // end, all stored in `LinePosition`. Field value apart from baseline is
+      // used in one other place
+      // (`nsBlockFrame`) - if that goes away, this becomes a duplication that
+      // should be removed.
+      LinePosition kidPosition;
+      for (const auto* kid : aFrame->PrincipalChildList()) {
+        LinePosition position;
+        if (!GetFirstLinePosition(aWM, kid, &position)) {
+          continue;
+        }
+        if (position.mBaseline < kidPosition.mBaseline) {
+          kidPosition = position;
+        }
+      }
+      if (kidPosition.mBaseline != nscoord_MAX) {
+        *aResult = kidPosition;
+        return true;
+      }
+    }
+
     // No baseline.
     return false;
   }
@@ -5906,6 +5928,21 @@ bool nsLayoutUtils::GetLastLineBaseline(WritingMode aWM, const nsIFrame* aFrame,
       *aResult = std::clamp(*aResult, 0, maxBaseline);
       return true;
     }
+
+    // No need to duplicate the baseline logic (Unlike `GetFirstLinePosition`,
+    // we don't need to return any other value apart from baseline), just defer
+    // to `GetNaturalBaselineBOffset`. Technically, we could do this at
+    // `ColumnSetWrapperFrame` level, but this keeps it symmetric to
+    // `GetFirstLinePosition`.
+    if (aFrame->IsColumnSetFrame()) {
+      const auto baseline =
+          aFrame->GetNaturalBaselineBOffset(aWM, BaselineSharingGroup::Last);
+      if (!baseline) {
+        return false;
+      }
+      *aResult = aFrame->BSize(aWM) - *baseline;
+      return true;
+    }
     // No baseline.
     return false;
   }

layout/base/nsLayoutUtils.h

@@ -1754,7 +1754,9 @@ class nsLayoutUtils {
    * Otherwise returns false.
    */
   struct LinePosition {
-    nscoord mBStart, mBaseline, mBEnd;
+    nscoord mBStart{nscoord_MAX};
+    nscoord mBaseline{nscoord_MAX};
+    nscoord mBEnd{nscoord_MAX};
 
     LinePosition operator+(nscoord aOffset) const {
       LinePosition result;

layout/generic/ColumnSetWrapperFrame.cpp

@@ -232,6 +232,53 @@ nscoord ColumnSetWrapperFrame::GetPrefISize(gfxContext* aRenderingContext) {
   return iSize;
 }
 
+template <typename Iterator>
+Maybe<nscoord> ColumnSetWrapperFrame::GetBaselineBOffset(
+    Iterator aStart, Iterator aEnd, WritingMode aWM,
+    BaselineSharingGroup aBaselineGroup) const {
+  // Either forward iterator + first baseline, or reverse iterator + last
+  // baseline
+  MOZ_ASSERT((*aStart == PrincipalChildList().FirstChild() &&
+              aBaselineGroup == BaselineSharingGroup::First) ||
+                 (*aStart == PrincipalChildList().LastChild() &&
+                  aBaselineGroup == BaselineSharingGroup::Last),
+             "Iterator direction must match baseline sharing group.");
+  if (StyleDisplay()->IsContainLayout()) {
+    return Nothing{};
+  }
+
+  // Start from start/end of principal child list, and use the first valid
+  // baseline.
+  for (auto itr = aStart; itr != aEnd; ++itr) {
+    const nsIFrame* kid = *itr;
+    auto kidBaseline = kid->GetNaturalBaselineBOffset(aWM, aBaselineGroup);
+    if (!kidBaseline) {
+      continue;
+    }
+    // Baseline is offset from the kid's rectangle, so find the offset to the
+    // kid's rectangle.
+    LogicalRect kidRect{aWM, kid->GetLogicalNormalPosition(aWM, GetSize()),
+                        kid->GetLogicalSize(aWM)};
+    if (aBaselineGroup == BaselineSharingGroup::First) {
+      *kidBaseline += kidRect.BStart(aWM);
+    } else {
+      *kidBaseline += (GetLogicalSize().BSize(aWM) - kidRect.BEnd(aWM));
+    }
+    return kidBaseline;
+  }
+  return Nothing{};
+}
+
+Maybe<nscoord> ColumnSetWrapperFrame::GetNaturalBaselineBOffset(
+    WritingMode aWM, BaselineSharingGroup aBaselineGroup) const {
+  if (aBaselineGroup == BaselineSharingGroup::First) {
+    return GetBaselineBOffset(PrincipalChildList().cbegin(),
+                              PrincipalChildList().cend(), aWM, aBaselineGroup);
+  }
+  return GetBaselineBOffset(PrincipalChildList().crbegin(),
+                            PrincipalChildList().crend(), aWM, aBaselineGroup);
+}
+
 #ifdef DEBUG
 
 /* static */

layout/generic/ColumnSetWrapperFrame.h

@@ -59,6 +59,9 @@ class ColumnSetWrapperFrame final : public nsBlockFrame {
 
   nscoord GetPrefISize(gfxContext* aRenderingContext) override;
 
+  Maybe<nscoord> GetNaturalBaselineBOffset(
+      WritingMode aWM, BaselineSharingGroup aBaselineGroup) const override;
+
  private:
   explicit ColumnSetWrapperFrame(ComputedStyle* aStyle,
                                  nsPresContext* aPresContext);
@@ -71,6 +74,11 @@ class ColumnSetWrapperFrame final : public nsBlockFrame {
   // its descendants.
   bool mFinishedBuildingColumns = false;
 #endif
+
+  template <typename Iterator>
+  Maybe<nscoord> GetBaselineBOffset(Iterator aStart, Iterator aEnd,
+                                    WritingMode aWM,
+                                    BaselineSharingGroup aBaselineGroup) const;
 };
 
 }  // namespace mozilla

layout/generic/nsColumnSetFrame.cpp

@@ -15,6 +15,7 @@
 #include "mozilla/ToString.h"
 #include "nsCSSRendering.h"
 #include "nsDisplayList.h"
+#include "nsIFrameInlines.h"
 #include "nsLayoutUtils.h"
 
 using namespace mozilla;
@@ -1295,6 +1296,32 @@ void nsColumnSetFrame::AppendDirectlyOwnedAnonBoxes(
   aResult.AppendElement(OwnedAnonBox(column));
 }
 
+Maybe<nscoord> nsColumnSetFrame::GetNaturalBaselineBOffset(
+    WritingMode aWM, BaselineSharingGroup aBaselineGroup) const {
+  Maybe<nscoord> result;
+  for (const auto* kid : mFrames) {
+    auto kidBaseline = kid->GetNaturalBaselineBOffset(aWM, aBaselineGroup);
+    if (!kidBaseline) {
+      continue;
+    }
+    // The kid frame may not necessarily be aligned with the columnset frame.
+    LogicalRect kidRect{aWM, kid->GetLogicalNormalPosition(aWM, GetSize()),
+                        kid->GetLogicalSize(aWM)};
+    if (aBaselineGroup == BaselineSharingGroup::First) {
+      *kidBaseline += kidRect.BStart(aWM);
+    } else {
+      *kidBaseline += (GetLogicalSize().BSize(aWM) - kidRect.BEnd(aWM));
+    }
+    // Take the smallest of the baselines (i.e. Closest to border-block-start
+    // for `BaselineSharingGroup::First`, border-block-end for
+    // `BaselineSharingGroup::Last`)
+    if (!result || *kidBaseline < *result) {
+      result = kidBaseline;
+    }
+  }
+  return result;
+}
+
 #ifdef DEBUG
 void nsColumnSetFrame::SetInitialChildList(ChildListID aListID,
                                            nsFrameList&& aChildList) {

layout/generic/nsColumnSetFrame.h

@@ -74,6 +74,10 @@ class nsColumnSetFrame final : public nsContainerFrame {
                              gfxContext* aCtx, const nsRect& aDirtyRect,
                              const nsPoint& aPt);
 
+  Maybe<nscoord> GetNaturalBaselineBOffset(
+      mozilla::WritingMode aWM,
+      BaselineSharingGroup aBaselineGroup) const override;
+
  protected:
   nscoord mLastBalanceBSize;
   nsReflowStatus mLastFrameStatus;

testing/web-platform/meta/css/css-multicol/baseline-001.html.ini

@@ -1,2 +0,0 @@
-[baseline-001.html]
-  expected: FAIL

testing/web-platform/meta/css/css-multicol/baseline-002.html.ini

@@ -1,2 +0,0 @@
-[baseline-002.html]
-  expected: FAIL

testing/web-platform/meta/css/css-multicol/baseline-008.html.ini

@@ -1,2 +0,0 @@
-[baseline-008.html]
-  expected: FAIL