Bug 64321 – Changing url cursor at runtime causes flickering

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Ananta Iyengar]

This bug was originally logged as a Chromium bug. http://code.google.com/p/chromium/issues/detail?id=87488.

The steps copied from the chromium report are as below:-

1. create a custom, animating cursor with a canvas
2. update the cursor w/ document.style.cursor = 'url('+canvas.toDataURL()+'), pointer' (do this repeatedly)
3. notice occasional flickering where the cursor is replaced with the default for a very brief moment before the new custom cursor is rendered.

Some debugging revealed that the flicker happens because the code to set the cursor runs at times before the webkit cursor cache is populated
with the new cursor image. As a result the cursor reverts back to the default pointer cursor for a brief moment, thus causing the flicker.

Ideas from Darin Fisher:- 

1. Webkit could use the previous cursor image while the new cursor is fetched.
2. Data urls could be processed synchronously.

[Nate Chapin]

Created attachment 100686 [details]
patch

[WebKit Review Bot]

Comment on attachment 100686 [details]
patch

Clearing flags on attachment: 100686

Committed r91003: <http://trac.webkit.org/changeset/91003>

[WebKit Review Bot]

All reviewed patches have been landed.  Closing bug.

[Oleg Romashin (:romaxa)]

With this fix I see crash in WebKit2 [Qt]

#0  0xb55e880d in WebKit::ShareableBitmap::createQImage (this=0x0)
    at ../../../Source/WebKit2/Shared/qt/ShareableBitmapQt.cpp:42
#1  0xb55e89c1 in WebKit::ShareableBitmap::createGraphicsContext (this=0x0)
    at ../../../Source/WebKit2/Shared/qt/ShareableBitmapQt.cpp:56
#2  0xb55d0692 in CoreIPC::encodeImage (encoder=0xacc28758, image=0x9786a58)
    at ../../../Source/WebKit2/Shared/WebCoreArgumentCoders.cpp:294
#3  0xb55d0927 in CoreIPC::ArgumentCoder<WebCore::Cursor>::encode (
    encoder=0xacc28758, cursor=...)
    at ../../../Source/WebKit2/Shared/WebCoreArgumentCoders.cpp:324
#4  0xb563a720 in CoreIPC::ArgumentEncoder::encode<WebCore::Cursor> (
    this=0xacc28758, t=...)
    at ../../../Source/WebKit2/Platform/CoreIPC/ArgumentEncoder.h:66
#5  0xb563a634 in CoreIPC::Arguments1<WebCore::Cursor const&>::encode (
    this=0xbfd4bf9c, encoder=0xacc28758)
    at ../../../Source/WebKit2/Platform/CoreIPC/Arguments.h:72
#6  0xb563a3b5 in CoreIPC::ArgumentCoder<Messages::WebPageProxy::SetCursor>::encode (encoder=0xacc28758, t=...)
    at ../../../Source/WebKit2/Platform/CoreIPC/ArgumentCoder.h:39
#7  0xb5639ed6 in CoreIPC::ArgumentEncoder::encode<Messages::WebPageProxy::SetCursor> (this=0xacc28758, t=...)
    at ../../../Source/WebKit2/Platform/CoreIPC/ArgumentEncoder.h:66
#8  0xb5638fe8 in CoreIPC::MessageSender<WebKit::WebPage>::send<Messages::WebPageProxy::SetCursor> (this=0xb0c00b10, message=..., destinationID=1)
---Type <return> to continue, or q <return> to quit---
    at ../../../Source/WebKit2/Platform/CoreIPC/MessageSender.h:44
#9  0xb56378f7 in CoreIPC::MessageSender<WebKit::WebPage>::send<Messages::WebPageProxy::SetCursor> (this=0xb0c00b10, message=...)
    at ../../../Source/WebKit2/Platform/CoreIPC/MessageSender.h:38
#10 0xb56352d3 in WebKit::WebChromeClient::setCursor (this=0xb0c00488, 
    cursor=...)
    at ../../../Source/WebKit2/WebProcess/WebCoreSupport/WebChromeClient.cpp:648
#11 0xb5b0f9b7 in WebCore::Chrome::setCursor (this=0xb0c00fb8, cursor=...)
    at ../../../Source/WebCore/page/Chrome.cpp:487
#12 0xb5dd9d99 in QXmlStreamAttribute::namespaceUri (this=0xbfd4c0a0)
    at /usr/include/qt4/QtCore/qxmlstream.h:148
#13 0xb5b3862f in WebCore::EventHandler::handleMouseReleaseEvent (
    this=0x9668f44, mouseEvent=...)
    at ../../../Source/WebCore/page/EventHandler.cpp:1718
#14 0xb5b37e87 in WebCore::EventHandler::handleMouseMoveEvent (this=0x9668f44, 
    mouseEvent=..., hoveredNode=0xbfd4c178)
    at ../../../Source/WebCore/page/EventHandler.cpp:1636
#15 0xb5578f14 in WebKit::handleMouseEvent (mouseEvent=..., page=0xb0c00f00)
    at ../../../Source/WebKit2/WebProcess/WebPage/WebPage.cpp:1053
#16 0xb5579034 in WebKit::WebPage::mouseEvent (this=0xb0c00b10, mouseEvent=...)
    at ../../../Source/WebKit2/WebProcess/WebPage/WebPage.cpp:1079
#17 0xb559f779 in CoreIPC::callMemberFunction<WebKit::WebPage, void (WebKit::Web---Type <return> to continue, or q <return> to quit---
Page::*)(WebKit::WebMouseEvent const&), WebKit::WebMouseEvent> (args=..., 
    object=0xb0c00b10, function=
    (void (WebKit::WebPage::*)(WebKit::WebPage *, const WebKit::WebMouseEvent &)) 0xb5578f60 <WebKit::WebPage::mouseEvent(WebKit::WebMouseEvent const&)>)
    at ../../../Source/WebKit2/Platform/CoreIPC/HandleMessage.h:19
#18 0xb559d553 in CoreIPC::handleMessage<Messages::WebPage::MouseEvent, WebKit::WebPage, void (WebKit::WebPage::*)(WebKit::WebMouseEvent const&)> (
    argumentDecoder=0x9a18628, object=0xb0c00b10, function=
    (void (WebKit::WebPage::*)(WebKit::WebPage *, const WebKit::WebMouseEvent &)) 0xb5578f60 <WebKit::WebPage::mouseEvent(WebKit::WebMouseEvent const&)>)
    at ../../../Source/WebKit2/Platform/CoreIPC/HandleMessage.h:277
#19 0xb559be44 in WebKit::WebPage::didReceiveWebPageMessage (this=0xb0c00b10, 
    messageID=..., arguments=0x9a18628)
    at generated/WebPageMessageReceiver.cpp:104
#20 0xb557d00f in WebKit::WebPage::didReceiveMessage (this=0xb0c00b10, 
    connection=0x963a528, messageID=..., arguments=0x9a18628)
    at ../../../Source/WebKit2/WebProcess/WebPage/WebPage.cpp:2086
#21 0xb558950b in WebKit::WebProcess::didReceiveMessage (this=0x963a190, 
    connection=0x963a528, messageID=..., arguments=0x9a18628)
    at ../../../Source/WebKit2/WebProcess/WebProcess.cpp:641
#22 0xb55b80bc in CoreIPC::Connection::dispatchMessage (this=0x963a528, 
    message=...) at ../../../Source/WebKit2/Platform/CoreIPC/Connection.cpp:677
#23 0xb55b8265 in CoreIPC::Connection::dispatchMessages (this=0x963a528)
---Type <return> to continue, or q <return> to quit---
    at ../../../Source/WebKit2/Platform/CoreIPC/Connection.cpp:704
#24 0xb55c0a21 in MemberFunctionWorkItem0<CoreIPC::Connection>::execute (
    this=0x99f6c10) at ../../../Source/WebKit2/Platform/WorkItem.h:79
#25 0xb54be18d in RunLoop::performWork (this=0x9638048)
    at ../../../Source/WebKit2/Platform/RunLoop.cpp:63
#26 0xb54bf164 in RunLoop::TimerObject::performWork (this=0x9525840)
    at ../../../Source/WebKit2/Platform/qt/RunLoopQt.cpp:49
#27 0xb54bfc16 in RunLoop::TimerObject::qt_metacall (this=0x9525840, 
    _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x98f49d8)
    at ./RunLoopQt.moc:71
#28 0xb2a36e4d in QMetaObject::metacall(QObject*, QMetaObject::Call, int, void**) () from /usr/lib/libQtCore.so.4
#29 0xb2a41795 in QMetaCallEvent::placeMetaCall(QObject*) ()
   from /usr/lib/libQtCore.so.4
#30 0xb2a48caf in QObject::event(QEvent*) () from /usr/lib/libQtCore.so.4
#31 0xb2e090a4 in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
   from /usr/lib/libQtGui.so.4
#32 0xb2e0e432 in QApplication::notify(QObject*, QEvent*) ()
   from /usr/lib/libQtGui.so.4
#33 0xb2a30a9e in QCoreApplication::notifyInternal(QObject*, QEvent*) ()
   from /usr/lib/libQtCore.so.4
#34 0xb2a34264 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () from /usr/lib/libQtCore.so.4

[Oleg Romashin (:romaxa)]

crash is happening on maps.google.com... by reverting this fix, don't see any flickering..

Should I add check for NULL somewhere in WebKit2 code(in new bug) or do something with this bug?

[Darin Fisher (:fishd, Google)]

Looks like CoreIPC::encodeImage may need a null check.

[Oleg Romashin (:romaxa)]

(In reply to comment #6)
> Looks like CoreIPC::encodeImage may need a null check.

Patches attached in new bug:
https://bugs.webkit.org/show_bug.cgi?id=65420