Bug 1777627 Part 1 - Improve BlockReflowState::IsAdjacentWithTop(). r=emilio

I rename and add a comment to the helper, and rewrite it to make it closer to
what the comment describes. The behavior is the same because `mBCoord` and
`mContentArea.BStart(wm)` are both initialized as `mBorderPadding.BStart(wm)`
[1].

Note `isAdjacentWithTop` local variable in ReflowFloat() is not changed because
it's going to be removed in the next patch.

[1] https://searchfox.org/mozilla-central/rev/2005e8d87ee045f19dac58e5bff32eff7d01bc9b/layout/generic/BlockReflowState.cpp#142

Differential Revision: https://phabricator.services.mozilla.com/D150831

layout/generic/BlockReflowState.cpp

@@ -781,7 +781,8 @@ bool BlockReflowState::FlowAndPlaceFloat(nsIFrame* aFloat) {
   // Are we required to place at least part of the float because we're
   // at the top of the page (to avoid an infinite loop of pushing and
   // breaking).
-  bool mustPlaceFloat = mReflowInput.mFlags.mIsTopOfPage && IsAdjacentWithTop();
+  bool mustPlaceFloat =
+      mReflowInput.mFlags.mIsTopOfPage && IsAdjacentWithBStart();
 
   for (;;) {
     if (mReflowInput.AvailableHeight() != NS_UNCONSTRAINEDSIZE &&

layout/generic/BlockReflowState.h

@@ -195,9 +195,10 @@ class BlockReflowState {
       nsIFrame* aFloatAvoidingBlock,
       const nsFlowAreaRect& aFloatAvailableSpace) const;
 
-  bool IsAdjacentWithTop() const {
-    return mBCoord == mBorderPadding.BStart(mReflowInput.GetWritingMode());
-  }
+  // True if the current block-direction coordinate, for placing the children
+  // within the content area, is still adjacent with the block-start of the
+  // content area.
+  bool IsAdjacentWithBStart() const { return mBCoord == ContentBStart(); }
 
   /**
    * Return mBlock's computed physical border+padding with GetSkipSides applied.

layout/generic/nsBlockFrame.cpp

@@ -3534,7 +3534,7 @@ bool nsBlockFrame::ShouldApplyBStartMargin(BlockReflowState& aState,
     return true;
   }
 
-  if (!aState.IsAdjacentWithTop()) {
+  if (!aState.IsAdjacentWithBStart()) {
     // If we aren't at the start block-coordinate then something of non-zero
     // height must have been placed. Therefore the childs block-start margin
     // applies.
@@ -3904,7 +3904,7 @@ void nsBlockFrame::ReflowBlockFrame(BlockReflowState& aState,
 
       frameReflowStatus.Reset();
       brc.ReflowBlock(availSpace, applyBStartMargin, aState.mPrevBEndMargin,
-                      clearance, aState.IsAdjacentWithTop(), aLine.get(),
+                      clearance, aState.IsAdjacentWithBStart(), aLine.get(),
                       *childReflowInput, frameReflowStatus, aState);
 
       if (frameReflowStatus.IsInlineBreakBefore()) {
@@ -4048,7 +4048,7 @@ void nsBlockFrame::ReflowBlockFrame(BlockReflowState& aState,
       // Don't force the block to fit if it's impacted by a float. If it is,
       // then pushing it to the next page would give it more room. Note that
       // isImpacted doesn't include impact from the block's own floats.
-      bool forceFit = aState.IsAdjacentWithTop() && clearance <= 0 &&
+      bool forceFit = aState.IsAdjacentWithBStart() && clearance <= 0 &&
                       !floatAvailableSpace.HasFloats();
       nsCollapsingMargin collapsedBEndMargin;
       OverflowAreas overflowAreas;
@@ -6688,7 +6688,7 @@ void nsBlockFrame::ReflowFloat(BlockReflowState& aState,
   nsBlockReflowContext brc(aState.mPresContext, aState.mReflowInput);
 
   // Reflow the float
-  bool isAdjacentWithTop = aState.IsAdjacentWithTop();
+  bool isAdjacentWithTop = aState.IsAdjacentWithBStart();
 
   nsIFrame* clearanceFrame = nullptr;
   do {