Previously this issue was hidden because LayoutSVGRoot::clippedOverflowRectForPaintInvalidation() explicitly calls LayoutReplaced::mapToVisibleRectInAncestorSpace() instead of its own mapToVisibleRectInAncestorSpace().

Is it hard to fix?

Not hard for fixing issue itself.

Discussed with trchen@ and he had an idea to separate mapToVisibleRectInAncestorSpace() (and other mapping functions) into two parts:
1. map from object local space (e.g. border box space) to container's contents space (e.g. scrolling box space);
2. map from contents space to local space.

Currently 2 for the container is called in child's mapToVisibleRectInAncestorSpace(), but with some exceptions which are wrong (e.g. LayoutSVGRoot). This requires the each child having its own way to do 1 to also know how to do 2 for the container. It would be clearer and less error-prone if we separate 2 into a method of the container objects. Trchen@ might work on this after my paint invalidation CLs are landed.

However, will GeometryMapper cover such things?

GeometryMapper does not cover this. GeometryMapper requires inputs (and outputs) that are in a well-defined property tree state, which means the space that SPv2 paintOffset is relative to: the space of the containing transform node.

My current work will cover this bug.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/afa2305acd724e42643af1fcf0a9807be4f3df99

commit afa2305acd724e42643af1fcf0a9807be4f3df99
Author: wangxianzhu <wangxianzhu@chromium.org>
Date: Tue Apr 05 00:37:03 2016

Don't apply svg viewport clip when mapping a LayoutSVGRoot's own rect

BUG= 597813 

Review URL: https://codereview.chromium.org/1852183002

Cr-Commit-Position: refs/heads/master@{#385064}

[modify] https://crrev.com/afa2305acd724e42643af1fcf0a9807be4f3df99/third_party/WebKit/Source/core/core.gypi
[modify] https://crrev.com/afa2305acd724e42643af1fcf0a9807be4f3df99/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
[modify] https://crrev.com/afa2305acd724e42643af1fcf0a9807be4f3df99/third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp
[modify] https://crrev.com/afa2305acd724e42643af1fcf0a9807be4f3df99/third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.h
[add] https://crrev.com/afa2305acd724e42643af1fcf0a9807be4f3df99/third_party/WebKit/Source/core/layout/svg/LayoutSVGRootTest.cpp
[modify] https://crrev.com/afa2305acd724e42643af1fcf0a9807be4f3df99/third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.cpp
[modify] https://crrev.com/afa2305acd724e42643af1fcf0a9807be4f3df99/third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.h