Collect active stylesheets and and apply asynchronously.

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 8f14f007e448d32c2a6764b3b3b04a6adad430ce..bb6ac017c8b5dba124d3ff07d326c3e32bc86ee7 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -30,7 +30,6 @@
 #include "core/MediaTypeNames.h"
 #include "core/animation/DocumentAnimations.h"
 #include "core/css/FontFaceSet.h"
-#include "core/css/resolver/StyleResolver.h"
 #include "core/dom/AXObjectCache.h"
 #include "core/dom/DOMNodeIds.h"
 #include "core/dom/ElementVisibilityObserver.h"
@@ -38,6 +37,7 @@
 #include "core/dom/IntersectionObserverCallback.h"
 #include "core/dom/IntersectionObserverController.h"
 #include "core/dom/IntersectionObserverInit.h"
+#include "core/dom/StyleChangeReason.h"
 #include "core/dom/TaskRunnerHelper.h"
 #include "core/editing/EditingUtilities.h"
 #include "core/editing/FrameSelection.h"
@@ -1463,6 +1463,10 @@ void FrameView::adjustMediaTypeForPrinting(bool printing) {
       setMediaType(m_mediaTypeWhenNotPrinting);
     m_mediaTypeWhenNotPrinting = nullAtom;
   }
+
+  m_frame->document()->setNeedsStyleRecalc(

[esprehn, 2016/12/09 01:36:21]

Ouch, did this code path always force a full document recalc?

[rune, 2016/12/09 08:55:50]

Yes, that is, setMediaType() called from here calls
mediaQueryAffectingValueChanged() which used to cause a full document recalc
regardlessly through resolverChanged(FullStyleUpdate).
mediaQueryAffectingValueChanged() now only has an effect if media queries
actually did change evaluation. If I leave this out, I'll have regressions in
tests for printing and zoom, at least. That is because of [1]. There might be
other switches on document().printing() which currently also requires
recalc/relayout if zoom factor is different from 1. We can surely improve on
this, but I think that could be a separate task.

Perhaps the setNeedsStyleRecalc() call here should be moved to setMediaType? I
looks like we only call setMediaType for switching between print and the other
media type (always screen?).

[1]
https://codesearch.chromium.org/chromium/src/third_party/WebKit/Source/core/c...

+      SubtreeStyleChange,
+      StyleChangeReasonForTracing::create(StyleChangeReason::StyleSheetChange));
 }
 
 bool FrameView::contentsInCompositedLayer() const {