Bug 1731541 - Implement text-wrap: balance for nsBlockFrame reflow. r=emilio

A simple form of balance for short blocks, implemented by incrementally reducing
the effective inline-size used during line-breaking, up to the point where an
extra line would be created.

This fails the test text-wrap-balance-line-clamp-001.html, but it's unclear to me
if that test is correct (see w3c/csswg-drafts#9310).
If we do want the behavior expected by that test, an additional patch to handle
the interaction with line-clamp will be required.

Depends on D187543

Differential Revision: https://phabricator.services.mozilla.com/D187544

Author: jfkthame

layout/generic/BlockReflowState.cpp

@@ -27,17 +27,16 @@
 using namespace mozilla;
 using namespace mozilla::layout;
 
-BlockReflowState::BlockReflowState(const ReflowInput& aReflowInput,
-                                   nsPresContext* aPresContext,
-                                   nsBlockFrame* aFrame, bool aBStartMarginRoot,
-                                   bool aBEndMarginRoot,
-                                   bool aBlockNeedsFloatManager,
-                                   const nscoord aConsumedBSize,
-                                   const nscoord aEffectiveContentBoxBSize)
+BlockReflowState::BlockReflowState(
+    const ReflowInput& aReflowInput, nsPresContext* aPresContext,
+    nsBlockFrame* aFrame, bool aBStartMarginRoot, bool aBEndMarginRoot,
+    bool aBlockNeedsFloatManager, const nscoord aConsumedBSize,
+    const nscoord aEffectiveContentBoxBSize, const nscoord aInset)
     : mBlock(aFrame),
       mPresContext(aPresContext),
       mReflowInput(aReflowInput),
       mContentArea(aReflowInput.GetWritingMode()),
+      mInsetForBalance(aInset),
       mPushedFloats(nullptr),
       mOverflowTracker(nullptr),
       mBorderPadding(

layout/generic/BlockReflowState.h

@@ -98,7 +98,8 @@ class BlockReflowState {
                    nsBlockFrame* aFrame, bool aBStartMarginRoot,
                    bool aBEndMarginRoot, bool aBlockNeedsFloatManager,
                    const nscoord aConsumedBSize,
-                   const nscoord aEffectiveContentBoxBSize);
+                   const nscoord aEffectiveContentBoxBSize,
+                   const nscoord aInset = 0);
 
   /**
    * Get the available reflow space (the area not occupied by floats)
@@ -243,9 +244,9 @@ class BlockReflowState {
   // the block.
 
   // The block frame that is using this object
-  nsBlockFrame* mBlock;
+  nsBlockFrame* const mBlock;
 
-  nsPresContext* mPresContext;
+  nsPresContext* const mPresContext;
 
   const ReflowInput& mReflowInput;
 
@@ -301,6 +302,10 @@ class BlockReflowState {
     return mContentArea.Size(wm).ConvertTo(aWM, wm);
   }
 
+  // Amount of inset to apply during line-breaking, used by text-wrap:balance
+  // to adjust line-breaks for more consistent lengths throughout the block.
+  nscoord mInsetForBalance;
+
   // Physical size. Use only for physical <-> logical coordinate conversion.
   nsSize mContainerSize;
   const nsSize& ContainerSize() const { return mContainerSize; }
@@ -345,7 +350,7 @@ class BlockReflowState {
 
   // mBlock's computed logical border+padding with pre-reflow skip sides applied
   // (See the constructor and nsIFrame::PreReflowBlockLevelLogicalSkipSides).
-  LogicalMargin mBorderPadding;
+  const LogicalMargin mBorderPadding;
 
   // The overflow areas of all floats placed so far
   OverflowAreas mFloatOverflowAreas;
@@ -383,7 +388,7 @@ class BlockReflowState {
   // placed, since we're on a nowrap context.
   nsTArray mNoWrapFloats;
 
-  nscoord mMinLineHeight;
+  const nscoord mMinLineHeight;
 
   int32_t mLineNumber;