Fix percent height calc for svg docs.

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

Index: third_party/WebKit/Source/core/layout/LayoutView.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutView.cpp b/third_party/WebKit/Source/core/layout/LayoutView.cpp
index 343bec145d51e2d224f9cf1f4b36e1d2322d635b..030045dc0256ace612c9c0346c96c6d19eac7968 100644
--- a/third_party/WebKit/Source/core/layout/LayoutView.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutView.cpp
@@ -280,6 +280,33 @@ void LayoutView::UpdateLayout() {
       !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;