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

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
 #include "core/paint/FilterPainter.h"
@@ skipping 817 matching lines @@
     PaintLayerFlags paint_flags,
     const LayoutPoint& fragment_translation) {
   // This involves subtracting out the position of the layer in our current
   // coordinate space, but preserving the accumulated error for sub-pixel
   // layout.
   LayoutPoint delta;
   paint_layer_.ConvertToLayerCoords(painting_info.root_layer, delta);
   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.
   FloatPoint3D transform_origin;
   Transform3DRecorder transform3d_recorder(
       context, paint_layer_.GetLayoutObject(),
       DisplayItem::kTransform3DElementTransform, transform, transform_origin);
 
   // Now do a paint with the root layer shifted to be us.
   PaintLayerPaintingInfo transformed_painting_info(
@@ skipping 404 matching lines @@
           context, layout_object, kPaintPhaseClippingMask))
     return;
 
   IntRect snapped_clip_rect = PixelSnappedIntRect(clip_rect.Rect());
   LayoutObjectDrawingRecorder drawing_recorder(
       context, layout_object, kPaintPhaseClippingMask, snapped_clip_rect);
   context.FillRect(snapped_clip_rect, Color::kBlack);
 }
 
 }  // namespace blink