Don't always fully repaint on viewport resize

Source/core/rendering/style/FillLayer.cpp

Index: Source/core/rendering/style/FillLayer.cpp
diff --git a/Source/core/rendering/style/FillLayer.cpp b/Source/core/rendering/style/FillLayer.cpp
index 907a80d5b52cf5884b11f9d920229f42dc620e42..70a0e1cf25a0e93d5e60241d3f59298c83f747e6 100644
--- a/Source/core/rendering/style/FillLayer.cpp
+++ b/Source/core/rendering/style/FillLayer.cpp
@@ -355,6 +355,42 @@ bool FillLayer::imagesAreLoaded() const
     return true;
 }
 
+bool FillLayer::needsFullRepaintOnContainerWidthChange() const
+{
+    if (m_image) {
+        if (m_repeatX == RoundFill || m_repeatX == SpaceFill)
+            return true;
+        if (m_xPosition.isPercent() && !m_xPosition.isZero())

[esprehn, 2014/05/08 20:42:03]

ditto

+            return true;
+        if (m_backgroundXOrigin != LeftEdge)
+            return true;
+        if (m_sizeType == SizeLength && m_sizeLength.width().isPercent())

[esprehn, 2014/05/08 20:42:03]

Why don't you check 0% here?

+            return true;
+        if (m_sizeType == Contain || m_sizeType == Cover)
+            return true;
+    }
+
+    return m_next && m_next->needsFullRepaintOnContainerWidthChange();
+}
+
+bool FillLayer::needsFullRepaintOnContainerHeightChange() const
+{
+    if (m_image) {
+        if (m_repeatY == RoundFill || m_repeatY == SpaceFill)
+            return true;
+        if (m_yPosition.isPercent() && !m_yPosition.isZero())

[esprehn, 2014/05/08 20:42:03]

Why do we ever represent 0 as a percent? It seems like 0% should be identical to
0px

[Xianzhu, 2014/05/08 22:31:08]

For visual effect, 0% and 0px are the same, but for internal representation they
are different. 0% is the default value and should not repaint on size change.

+            return true;
+        if (m_backgroundYOrigin != TopEdge)
+            return true;
+        if (m_sizeType == SizeLength && m_sizeLength.height().isPercent())

[esprehn, 2014/05/08 20:42:03]

why not check 0%?

[Xianzhu, 2014/05/08 22:31:08]

Omitted this because it's not a common usage and I was not sure about the actual
behavior. Just tested, background-size: 0% just hides the background image, so
doesn't need repaint.

Added.

+            return true;
+        if (m_sizeType == Contain || m_sizeType == Cover)
+            return true;

[esprehn, 2014/05/08 20:42:03]

Lists like this make me really nervous, this is how we get corner case bugs.

[Xianzhu, 2014/05/08 22:31:08]

In the new patch set I changed the 'A||B' lists to '!C||!D' lists, so that if
there is any new value of the enum, the new value just fall into the safe path
(doing repaint).

For the new patch set, I carefully checked css3-background again. The function
seems now safe and complete.

However, the rules still look complicated. Another method of avoiding this is to
just check style()->hasBackgroundImage() in RenderView and if yes doing a full
repaint. This is not optimal, but we could split such complex logic into later
independent changes. wdyt?

+    }
+
+    return m_next && m_next->needsFullRepaintOnContainerWidthChange();
+}
+
 bool FillLayer::hasOpaqueImage(const RenderObject* renderer) const
 {
     if (!m_image)