Avoid lossy effects of rounding by using contentBoxRect directly for plugin bounds.

third_party/WebKit/Source/web/WebPluginContainerImpl.cpp

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2014 Opera Software ASA. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 885 matching lines @@
         unclippedIntLocalRect = IntRect();
         return;
     }
 
     LayoutView* rootView = m_element->document().view()->layoutView();
     while (rootView->frame()->ownerLayoutObject())
         rootView = rootView->frame()->ownerLayoutObject()->view();
 
     LayoutBox* box = toLayoutBox(ownerElement->layoutObject());
 
-    // Plugin frameRects are in absolute space within their frame.
-    FloatRect frameRectInOwnerElementSpace = box->absoluteToLocalQuad(FloatRect(frameRect()), UseTransforms).boundingBox();
-
-    LayoutRect unclippedAbsoluteRect(frameRectInOwnerElementSpace);
+    // Note: frameRect() for this plugin is equal to contentBoxRect, mapped to the containing view space, and rounded off.
+    // See LayoutPart.cpp::updateWidgetGeometryInternal. To remove the lossy effect of rounding off, use contentBoxRect directly.
+    LayoutRect unclippedAbsoluteRect(box->contentBoxRect());
     box->mapToVisualRectInAncestorSpace(rootView, unclippedAbsoluteRect);
 
     // The frameRect is already in absolute space of the local frame to the plugin.
     windowRect = frameRect();
     // Map up to the root frame.
     LayoutRect layoutWindowRect =
         LayoutRect(m_element->document().view()->layoutView()->localToAbsoluteQuad(FloatQuad(FloatRect(frameRect())), TraverseDocumentBoundaries).boundingBox());
     // Finally, adjust for scrolling of the root frame, which the above does not take into account.
     layoutWindowRect.moveBy(-rootView->viewRect().location());
     windowRect = pixelSnappedIntRect(layoutWindowRect);
 
     LayoutRect layoutClippedLocalRect = unclippedAbsoluteRect;
     LayoutRect unclippedLayoutLocalRect = layoutClippedLocalRect;
     layoutClippedLocalRect.intersect(LayoutRect(rootView->frameView()->visibleContentRect()));
 
     // TODO(chrishtr): intentionally ignore transform, because the positioning of frameRect() does also. This is probably wrong.

[Xianzhu, 2016/07/07 18:49:57]

Nit: Remove the TODO.

[chrishtr, 2016/07/07 20:02:14]

Done.

-    unclippedIntLocalRect = box->absoluteToLocalQuad(FloatRect(unclippedLayoutLocalRect), TraverseDocumentBoundaries).enclosingBoundingBox();
+    unclippedIntLocalRect = box->absoluteToLocalQuad(FloatRect(unclippedLayoutLocalRect), TraverseDocumentBoundaries | UseTransforms).enclosingBoundingBox();
     // As a performance optimization, map the clipped rect separately if is different than the unclipped rect.
     if (layoutClippedLocalRect != unclippedLayoutLocalRect)
-        clippedLocalRect = box->absoluteToLocalQuad(FloatRect(layoutClippedLocalRect), TraverseDocumentBoundaries).enclosingBoundingBox();
+        clippedLocalRect = box->absoluteToLocalQuad(FloatRect(layoutClippedLocalRect), TraverseDocumentBoundaries | UseTransforms).enclosingBoundingBox();
     else
         clippedLocalRect = unclippedIntLocalRect;
 }
 
 void WebPluginContainerImpl::calculateGeometry(IntRect& windowRect, IntRect& clipRect, IntRect& unobscuredRect, Vector<IntRect>& cutOutRects)
 {
     // document().layoutView() can be null when we receive messages from the
     // plugins while we are destroying a frame.
     // FIXME: Can we just check m_element->document().isActive() ?
     if (m_element->layoutObject()->document().layoutView()) {
         // Take our element and get the clip rect from the enclosing layer and
         // frame view.
         computeClipRectsForPlugin(m_element, windowRect, clipRect, unobscuredRect);
     }
     getPluginOcclusions(m_element, this->parent(), frameRect(), cutOutRects);
     // Convert to the plugin position.
     for (size_t i = 0; i < cutOutRects.size(); i++)
         cutOutRects[i].move(-frameRect().x(), -frameRect().y());
 }
 
 } // namespace blink