added fix that deals with precision in layer sorter

cc/layer_sorter.cc

Index: cc/layer_sorter.cc
diff --git a/cc/layer_sorter.cc b/cc/layer_sorter.cc
index ef347dbe55b41f6ead7b16d804cda08d461477ed..8603b96c94db4b0ff0321616bf33fcfad6a0e31c 100644
--- a/cc/layer_sorter.cc
+++ b/cc/layer_sorter.cc
@@ -114,10 +114,24 @@ LayerSorter::ABCompareResult LayerSorter::checkOverlap(LayerShape* a, LayerShape
     // which layer is in front.
     float maxPositive = 0;
     float maxNegative = 0;
+
+    // flag to tell us if we've computed an accurate (w.r.t. floating point) answer

[danakj, 2012/11/01 20:15:33]

All comments should be complete sentences with capital letters and punctuation.

+    bool accurate = false;
     for (unsigned o = 0; o < overlapPoints.size(); o++) {
         float za = a->layerZFromProjectedPoint(overlapPoints[o]);
         float zb = b->layerZFromProjectedPoint(overlapPoints[o]);
 
+        // attempt to avoid issues with layers that are too close together
+        // if we have 2-sided quads that are very close together then we draw them

[enne (OOO), 2012/11/01 20:08:55]

style nit: cc code doesn't obey the Chromium 80 col limit yet in general, but if
you're going to wrap comments, at least wrap them at 80 col.

+        // in the order they arrived in to avoid z-fighting flickering
+        // The correct solution is for the content maker to turn on back-face culling
+        // or move the quads apart (if they're not two sides of one object)
+        float absdif = fabsf(zb - za);
+        float absmax = fmax(fabs(zb), fabs(za));

[enne (OOO), 2012/11/01 20:08:55]

style nit: Can you match the variable style in this file, i.e. absmax => absMax
and absdif => absDiff?

+        // check to see if we've got a result with a reasonable amount of error
+        if (absdif > 1e-4 * absmax)

[enne (OOO), 2012/11/01 20:08:55]

Can you pull this constant out into a named variable at the top of the file,
with some comment justification about why you picked this value?

+          accurate = true;
+
         float diff = za - zb;
         if (diff > maxPositive)
             maxPositive = diff;
@@ -125,6 +139,10 @@ LayerSorter::ABCompareResult LayerSorter::checkOverlap(LayerShape* a, LayerShape
             maxNegative = diff;
     }
 
+    // if we can't tell which should come first, use document order
+    if (!accurate)
+      return ABeforeB;
+
     float maxDiff = (fabsf(maxPositive) > fabsf(maxNegative) ? maxPositive : maxNegative);
 
     // If the results are inconsistent (and the z difference substantial to rule out