Reland of Don't pass subpixel offsets through non-translation transforms

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index cbfc784b6a080149d3e4c73a8f8bbfb774aff3b7..8f8df291a2cfd6ee58aef0aabae09fff9e61139a 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -835,10 +835,18 @@
   delta.MoveBy(fragment_translation);
   TransformationMatrix transform(
       paint_layer_.RenderableTransform(painting_info.GetGlobalPaintFlags()));
-  IntPoint rounded_delta = RoundedIntPoint(delta);
-  transform.PostTranslate(rounded_delta.X(), rounded_delta.Y());
-  LayoutSize adjusted_sub_pixel_accumulation =
-      painting_info.sub_pixel_accumulation + (delta - rounded_delta);
+  LayoutSize adjusted_sub_pixel_accumulation;
+  if (transform.IsIdentityOrTranslation()) {
+    IntPoint rounded_delta = RoundedIntPoint(delta);
+    transform.PostTranslate(rounded_delta.X(), rounded_delta.Y());
+    adjusted_sub_pixel_accumulation =
+        painting_info.sub_pixel_accumulation + (delta - rounded_delta);
+  } else {
+    // We can't pass subpixel offsets through a non-translation transform.
+    // Bake the offsets into the transform instead.
+    delta += painting_info.sub_pixel_accumulation;

[chrishtr, 2017/05/04 18:39:02]

Perhaps the bug here that caused issue 716163 is that
we failed to pixel-snap this post-translate. It does seem
incorrect to anchor the transform at a non-pixel-snapped
position.
I think that should fix it, could you confirm?

+    transform.PostTranslate(delta.X().ToDouble(), delta.Y().ToDouble());
+  }
 
   // TODO(jbroman): Put the real transform origin here, instead of using a
   // matrix with the origin baked in.