Fix percent height calc for svg docs.

third_party/WebKit/Source/core/layout/LayoutView.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc.
  *               All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 262 matching lines @@
   }
 
   SubtreeLayoutScope layout_scope(*this);
 
   // Use calcWidth/Height to get the new width/height, since this will take the
   // full page zoom factor into account.
   bool relayout_children =
       !ShouldUsePrintingLayout() &&
       (!frame_view_ || LogicalWidth() != ViewLogicalWidthForBoxSizing() ||
        LogicalHeight() != ViewLogicalHeightForBoxSizing());
+
+  // For a pure SVG doc, the svg root will have percentage height, which will be

[pdr., 2017/04/26 04:56:47]

Is this the tree we're considering?
frameview -> layoutview -> svgroot_with_percentage_height ->
svg_child_with_relative_lengths

If so, is this the chain of events?
1) begin layout, layoutview has 0,0 height
2) with existing code, relayout_children is false
3) layout the svg root
4) update the layout view size
5) update the frameview size
6) finish layout

[szager1, 2017/04/26 12:11:35]

Not exactly.  First layout is typically not a problem, because it winds up doing
additional layouts with the frame size stable.

The reason it shows up in the masking-clipping-hidpi.svg test is because the
FrameView size changes (due to the zoom level change) *after* the first layout. 
It goes like this:

WebViewImpl::ResizeViewWhileAnchored
  - FrameView::layout_size_ gets updated
  ...
  FrameView::UpdateLayout
    LayoutView::UpdateLayout
      - On this pass, relayout_children is true
    Document::LayoutUpdated()
      ChromeClientImpl::LayoutUpdated()
        WebViewImpl::LayoutUpdated()
          - FrameView::size_ gets updated
          FrameView::UpdateLayout
            LayoutView::UpdateLayout
              - On this pass, relayout_children is false

On the first layout pass, relayout_children is true because the
FrameView::LayoutSize() changed.  During this pass, the svg root resolves its
size against LayoutView::LogicalHeight() *before* the LayoutView calls
UpdateLogicalHeight().  So, it's based on the LayoutView's LogicalHeight from
the *previous* layout pass (i.e., the unzoomed height).  LayoutView runs
UpdateLogicalHeight *after* laying out its children; at that point, the
LayoutView's logical height is set based on (the new) FrameView::LayoutSize().

On the second layout pass, FrameView::Size() has changed, but relayout_children
is false because the FrameView::LayoutSize() did not change since the first
layout pass.  So, the svg root does not run UpdateLayout, and the new
LayoutView::LogicalHeight() from the first layout pass is never used to resolve
the svg root's height.

+  // resolved against the LayoutView's logical height, which comes from the
+  // FrameView's visible content height.  If the FrameView was resized during
+  // the previous layout, then the LayoutView's logical height would have
+  // changed *after* the svg root finished layout, so the logical height of
+  // the svg root would have been computed based on a stale container height.
+  // Check here whether the svg root's logical height matches what we would
+  // expect based on the LayoutView's now-up-to-date logical height.
+  if (!relayout_children && GetDocument().IsSVGDocument()) {

[pdr., 2017/04/26 04:56:47]

Can you use GetDocument().AccessSVGExtensions().rootElement() to simplify this?

Something like:
if (!relayout_children && GetDocument().IsSVGDocument()) {
  if (auto* svg_root = GetDocument().AccessSVGExtensions().rootElement()) {
    const Length& svg_root_height = svg_root->Style()->LogicalHeight();
    if (svg_root_height.IsPercentOrCalc()) {
      if (svg_root->LogicalHeight() !=
          ValueForLength(svg_root_height,
                        AvailableLogicalHeightUsing(
                            svg_root_height, kExcludeMarginBorderPadding))) {
        relayout_children = true;
        layout_scope.SetChildNeedsLayout(child);
      }
    }
  }
}

[szager1, 2017/04/26 12:11:35]

Yep, this works; I updated the patch.

+    for (LayoutObject* child = FirstChild(); child;
+         child = child->NextSibling()) {
+      if (!child->IsSVGRoot())
+        continue;
+      const Length& child_height = child->Style()->LogicalHeight();
+      if (!child_height.IsPercentOrCalc())
+        continue;
+      if (ToLayoutBox(child)->LogicalHeight() !=
+          ValueForLength(child_height,
+                         AvailableLogicalHeightUsing(
+                             child_height, kExcludeMarginBorderPadding))) {
+        relayout_children = true;
+        layout_scope.SetChildNeedsLayout(child);
+      }
+    }
+  }
+
   if (relayout_children) {
     layout_scope.SetChildNeedsLayout(this);
     for (LayoutObject* child = FirstChild(); child;
          child = child->NextSibling()) {
       if (child->IsSVGRoot())
         continue;
 
       if ((child->IsBox() && ToLayoutBox(child)->HasRelativeLogicalHeight()) ||
           child->Style()->LogicalHeight().IsPercentOrCalc() ||
           child->Style()->LogicalMinHeight().IsPercentOrCalc() ||
@@ skipping 521 matching lines @@
   // Frame scroll corner is painted using LayoutView as the display item client.
   if (!RuntimeEnabledFeatures::rootLayerScrollingEnabled() &&
       (GetFrameView()->HorizontalScrollbar() ||
        GetFrameView()->VerticalScrollbar()))
     return false;
 
   return LayoutBlockFlow::PaintedOutputOfObjectHasNoEffectRegardlessOfSize();
 }
 
 }  // namespace blink