CSS: SVG use elements replicate updates to style

third_party/WebKit/Source/core/dom/Node.cpp

Index: third_party/WebKit/Source/core/dom/Node.cpp
diff --git a/third_party/WebKit/Source/core/dom/Node.cpp b/third_party/WebKit/Source/core/dom/Node.cpp
index 49dbf9b8451b05d823c51a817960c9524f3005b2..a4d3370327286d3ab3f5f83be61315b2015751e1 100644
--- a/third_party/WebKit/Source/core/dom/Node.cpp
+++ b/third_party/WebKit/Source/core/dom/Node.cpp
@@ -89,6 +89,8 @@
 #include "core/layout/LayoutBox.h"
 #include "core/page/ContextMenuController.h"
 #include "core/page/Page.h"
+#include "core/svg/SVGElement.h"
+#include "core/svg/SVGUseElement.h"
 #include "core/svg/graphics/SVGImage.h"
 #include "platform/EventDispatchForbiddenScope.h"
 #include "platform/RuntimeEnabledFeatures.h"
@@ -700,11 +702,21 @@ void Node::setNeedsStyleRecalc(StyleChangeType changeType, const StyleChangeReas
     if (changeType > existingChangeType)
         setStyleChange(changeType);
 
-    if (existingChangeType == NoStyleChange)
-        markAncestorsWithChildNeedsStyleRecalc();
-
     if (isElementNode() && hasRareData())
         toElement(*this).setAnimationStyleChange(false);
+
+    if (existingChangeType != NoStyleChange)
+        return;
+    markAncestorsWithChildNeedsStyleRecalc();
+    if (!isSVGElement())
+        return;

[rune, 2016/08/18 21:15:28]

I think I'd move the code below into a separate method on SVGElement.

[Eric Willigers, 2016/08/18 22:27:00]

Done.

+    const HeapHashSet<WeakMember<SVGElement>>& set = toSVGElement(this)->instancesForElement();
+    if (set.isEmpty())
+        return;
+    for (SVGElement* instance : set) {
+        if (SVGUseElement* element = instance->correspondingUseElement())
+            element->setNeedsStyleRecalc(SubtreeStyleChange, reason);

[rune, 2016/08/18 21:09:07]

So there's an 1:n relationship between each svg element in the original subtree
and each and every svg element copy in the use shadow trees?

Could we make this cheaper than SubtreeStyleChange on the <use> element? Would
it be incorrect to just set the StyleChangeType on the specific instances
instead of nuking style for the the whole subtree for every change?

It's not possible to modify the use shadow tree to be different from the
referenced subtree, right?

IIUC, the computed styles for the copied subtree might be different because the
"used" elements will inherit from the <use> element. Is that right?

[fs, 2016/08/18 21:21:17]

Yes.
I was hoping we could do something like that. Haven't been able to come up with
a counter-example to that, but maybe Eric has?
Correct.
Yes.

[Eric Willigers, 2016/08/18 22:27:00]

Done. All my tests still pass. I should have tried it earlier.

+    }
 }
 
 void Node::clearNeedsStyleRecalc()