Don't un-flip if we're only mapping for one object.

third_party/WebKit/Source/core/layout/PaintInvalidationState.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/layout/PaintInvalidationState.h"
 
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/Settings.h"
 #include "core/layout/LayoutInline.h"
@@ skipping 396 matching lines @@
     if (m_paintInvalidationContainer->layer()->groupedMapping())
         PaintLayer::mapRectInPaintInvalidationContainerToBacking(*m_paintInvalidationContainer, rect);
     return rect;
 }
 
 static void slowMapToVisualRectInAncestorSpace(const LayoutObject& object, const LayoutBoxModelObject& ancestor, LayoutRect& rect)
 {
     // TODO(wkorman): The flip below is required because visual rects are
     // currently in "physical coordinates with flipped block-flow direction"
     // (see LayoutBoxModelObject.h) but we need them to be in physical
-    // coordinates.
-    if (object.isBox())
+    // coordinates. When we're mapping within exactly one object we keep the
+    // input rect flipped because we'd just have to re-flip in short order to
+    // produce the final result.

[chrishtr, 2016/09/07 22:47:23]

"because the output of this method must be in the "physical coordinates with
flipped block-flow direction" also

An even simpler way to write this patch could be:

if (object == ancestor)
  return;

[wkorman, 2016/09/07 22:52:28]

Conceptually yes (and I think we'd need an isBox() check?), but there is a small
bit of add'l logic we'd be skipping:

LayoutView::mapToVisualRectInAncestorSpace:
- adjust offset for fixed position (applies scroll/scrolledContent offset, I
think this would apply in some cases)
- apply transform for full page zooming (but only if !ancestor which isn't the
case)

LayoutBox::mapToVisualRectInAncestorSpace:
- inflate for reflection and filter (I think this would apply in some cases)

[chrishtr, 2016/09/07 22:54:02]

Right ok. Makes sense, these are indeed important.

[wkorman, 2016/09/07 23:18:18]

Updated comment.

+    if (object.isBox() && object != ancestor)
         toLayoutBox(&object)->flipForWritingMode(rect);
 
     if (object.isLayoutView())
         toLayoutView(object).mapToVisualRectInAncestorSpace(&ancestor, rect, InputIsInFrameCoordinates, DefaultVisualRectFlags);
     else
         object.mapToVisualRectInAncestorSpace(&ancestor, rect);
 }
 
 void PaintInvalidationState::mapLocalRectToPaintInvalidationContainer(LayoutRect& rect) const
 {
@@ skipping 111 matching lines @@
 }
 
 
 void PaintInvalidatorContextAdapter::mapLocalRectToPaintInvalidationBacking(const LayoutObject& object, LayoutRect& rect) const
 {
     DCHECK(&object == &m_paintInvalidationState.currentObject());
     m_paintInvalidationState.mapLocalRectToPaintInvalidationBacking(rect);
 }
 
 } // namespace blink