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

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

Index: third_party/WebKit/Source/core/layout/LayoutGrid.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
index c76dd7d4fd35df8cd800ba1964120a2db2a3be4e..bf51654c0818fef1014599cfa40a7a6537938922 100644
--- a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
@@ -565,6 +565,10 @@ LayoutUnit LayoutGrid::guttersSize(GridTrackSizingDirection direction, size_t st
             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)) {