Support slotchange events

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 832a32753ea3be8d7e1d126a70b68b588392669c..f1caf2f053eeafdd9b5f50f2e675ecbc741f5613 100644
--- a/third_party/WebKit/Source/core/dom/Node.cpp
+++ b/third_party/WebKit/Source/core/dom/Node.cpp
@@ -44,6 +44,7 @@
 #include "core/dom/ElementTraversal.h"
 #include "core/dom/ExceptionCode.h"
 #include "core/dom/LayoutTreeBuilderTraversal.h"
+#include "core/dom/Microtask.h"
 #include "core/dom/NodeRareData.h"
 #include "core/dom/NodeTraversal.h"
 #include "core/dom/ProcessingInstruction.h"
@@ -690,6 +691,11 @@ void Node::markAncestorsWithChildNeedsStyleInvalidation()
 void Node::markAncestorsWithChildNeedsDistributionRecalc()
 {
     ScriptForbiddenScope forbidScriptDuringRawIteration;
+    if (inDocument() && !document().childNeedsDistributionRecalc()) {
+        // TODO(hayato): Support a non-document composed tree.
+        // TODO(hayato): Enqueue a task only if a 'slotchange' event listner is registered in the document composed tree.
+        Microtask::enqueueMicrotask(WTF::bind(&Document::updateDistribution, PassRefPtrWillBeRawPtr<Document>(&document())));

[esprehn, 2016/02/18 17:30:43]

this means we force a distribution after every DOM mutation even if no one is
listening which is just wasted battery. Also this seems pretty bad, it means if
you do:

1) micro task that mutates the dom
2) microtask that dispatches an event (causes distribution), then mutates the
dom
3) microtask that dispatches another event (causes distribution), then mutates
the dom
4) now run *3* updateDistribution microtasks.

[esprehn, 2016/02/18 18:07:37]

This is also adding an extra microtask every time an input element or other
thing with a UA shadowroot is inserted since all of those also mark the document
for distribution.

ex.

.appendChild(input);
.dispatchEvent(...);
.appendChild(input);
.dispatchEvent(...);
.appendChild(input);
.dispatchEvent(...);
.appendChild(input);
// Now 3 no-op Document::updateDistribution microtasks run.

This grows the microtask list unbounded if you interleave creating elements that
use shadow roots and doing anything that forces distribution (events,
getComputedStyle, layout, ...)

That's actually exactly what <iron-list> in polymer does, they appendChild and
then measure this means your patch causes an extra 10 microtasks on the chrome
downloads page.

+    }
     for (Node* node = this; node && !node->childNeedsDistributionRecalc(); node = node->parentOrShadowHostNode())
         node->setChildNeedsDistributionRecalc();
     document().scheduleLayoutTreeUpdateIfNeeded();