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

third_party/WebKit/Source/web/WebPluginContainerImpl.cpp

Index: third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp b/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
index e143f82c3b2cb286e6c726780332bc6684822dee..0c43d08a067f6d4009692e1c4e8881240b2c3f35 100644
--- a/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
+++ b/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
@@ -903,10 +903,9 @@ void WebPluginContainerImpl::computeClipRectsForPlugin(
 
     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.
@@ -923,10 +922,10 @@ void WebPluginContainerImpl::computeClipRectsForPlugin(
     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;
 }