[css-grid] Too many gaps with trailing collapsing tracks

third_party/WebKit/Source/core/layout/LayoutGrid.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 547 matching lines @@
     // the grid (so the gap becomes 0) or there is a non empty track before that.
 
     LayoutUnit gapAccumulator;
     size_t endLine = startLine + span;
 
     for (size_t line = startLine; line < endLine - 1; ++line) {
         if (!isEmptyAutoRepeatTrack(direction, line))
             gapAccumulator += gap;
     }
 
+    // The above loop adds one extra gap for trailing collapsed tracks.
+    if (gapAccumulator && isEmptyAutoRepeatTrack(direction, endLine - 1))
+        gapAccumulator -= gap;

[eae, 2016/09/12 19:24:32]

Do we want to add a DCHECK here to ensure that it never goes below zero or is
that OK?

[Manuel Rego, 2016/09/12 19:50:17]

Right now "gapAccumulator > 0" implies that "gapAccumulator - gap"
won't be below zero. But I agree we could add the DCHECK to be safe
in future modifications of the code.

[svillar, 2016/09/13 07:36:27]

So gapAccumulator is only increased in "gap" chunks so checking gapAccumulator
should be enough.

+
     // If the startLine is the start line of a collapsed track we need to go backwards till we reach
     // a non collapsed track. If we find a non collapsed track we need to add that gap.
     if (startLine && isEmptyAutoRepeatTrack(direction, startLine)) {
         size_t nonEmptyTracksBeforeStartLine = startLine;
         auto begin = isRowAxis ? m_autoRepeatEmptyColumns->begin() : m_autoRepeatEmptyRows->begin();
         for (auto it = begin; *it != startLine; ++it) {
             DCHECK(nonEmptyTracksBeforeStartLine);
             --nonEmptyTracksBeforeStartLine;
         }
         if (nonEmptyTracksBeforeStartLine)
@@ skipping 2084 matching lines @@
     return isOrthogonalChild(child) ? childLocation.transposedPoint() : childLocation;
 }
 
 void LayoutGrid::paintChildren(const PaintInfo& paintInfo, const LayoutPoint& paintOffset) const
 {
     if (!m_gridItemArea.isEmpty())
         GridPainter(*this).paintChildren(paintInfo, paintOffset);
 }
 
 } // namespace blink