Planet Igalia WebKit

September 09, 2017

Carlos García Campos

WebDriver support in WebKitGTK+ 2.18

WebDriver is an automation API to control a web browser. It allows to create automated tests for web applications independently of the browser and platform. WebKitGTK+ 2.18, that will be released next week, includes an initial implementation of the WebDriver specification.

WebDriver in WebKitGTK+

There’s a new process (WebKitWebDriver) that works as the server, processing the clients requests to spawn and control the web browser. The WebKitGTK+ driver is not tied to any specific browser, it can be used with any WebKitGTK+ based browser, but it uses MiniBrowser as the default. The driver uses the same remote controlling protocol used by the remote inspector to communicate and control the web browser instance. The implementation is not complete yet, but it’s enough for what many users need.

The clients

The web application tests are the clients of the WebDriver server. The Selenium project provides APIs for different languages (Java, Python, Ruby, etc.) to write the tests. Python is the only language supported by WebKitGTK+ for now. It’s not yet upstream, but we hope it will be integrated soon. In the meantime you can use our fork in github. Let’s see an example to understand how it works and what we can do.

from selenium import webdriver

# Create a WebKitGTK driver instance. It spawns WebKitWebDriver 
# process automatically that will launch MiniBrowser.
wkgtk = webdriver.WebKitGTK()

# Let's load the WebKitGTK+ website.

# Find the GNOME link.
gnome = wkgtk.find_element_by_partial_link_text("GNOME")

# Click on the link.

# Find the search form. 
search = wkgtk.find_element_by_id("searchform")

# Find the first input element in the search form.
text_field = search.find_element_by_tag_name("input")

# Type epiphany in the search field and submit.

# Let's count the links in the contents div to check we got results.
contents = wkgtk.find_element_by_class_name("content")
links = contents.find_elements_by_tag_name("a")
assert len(links) > 0

# Quit the driver. The session is closed so MiniBrowser 
# will be closed and then WebKitWebDriver process finishes.

Note that this is just an example to show how to write a test and what kind of things you can do, there are better ways to achieve the same results, and it depends on the current source of public websites, so it might not work in the future.

Web browsers / applications

As I said before, WebKitWebDriver process supports any WebKitGTK+ based browser, but that doesn’t mean all browsers can automatically be controlled by automation (that would be scary). WebKitGTK+ 2.18 also provides new API for applications to support automation.

  • First of all the application has to explicitly enable automation using webkit_web_context_set_automation_allowed(). It’s important to know that the WebKitGTK+ API doesn’t allow to enable automation in several WebKitWebContexts at the same time. The driver will spawn the application when a new session is requested, so the application should enable automation at startup. It’s recommended that applications add a new command line option to enable automation, and only enable it when provided.
  • After launching the application the driver will request the browser to create a new automation session. The signal “automation-started” will be emitted in the context to notify the application that a new session has been created. If automation is not allowed in the context, the session won’t be created and the signal won’t be emitted either.
  • A WebKitAutomationSession object is passed as parameter to the “automation-started” signal. This can be used to provide information about the application (name and version) to the driver that will match them with what the client requires accepting or rejecting the session request.
  • The WebKitAutomationSession will emit the signal “create-web-view” every time the driver needs to create a new web view. The application can then create a new window or tab containing the new web view that should be returned by the signal. This signal will always be emitted even if the browser has already an initial web view open, in that case it’s recommened to return the existing empty web view.
  • Web views are also automation aware, similar to ephemeral web views, web views that allow automation should be created with the constructor property “is-controlled-by-automation” enabled.

This is the new API that applications need to implement to support WebDriver, it’s designed to be as safe as possible, but there are many things that can’t be controlled by WebKitGTK+, so we have several recommendations for applications that want to support automation:

  • Add a way to enable automation in your application at startup, like a command line option, that is disabled by default. Never allow automation in a normal application instance.
  • Enabling automation is not the only thing the application should do, so add an automation mode to your application.
  • Add visual feedback when in automation mode, like changing the theme, the window title or whatever that makes clear that a window or instance of the application is controllable by automation.
  • Add a message to explain that the window is being controlled by automation and the user is not expected to use it.
  • Use ephemeral web views in automation mode.
  • Use a temporal user profile in application mode, do not allow automation to change the history, bookmarks, etc. of an existing user.
  • Do not load any homepage in automation mode, just keep an empty web view (about:blank) that can be used when a new web view is requested by automation.

The WebKitGTK client driver

Applications need to implement the new automation API to support WebDriver, but the WebKitWebDriver process doesn’t know how to launch the browsers. That information should be provided by the client using the WebKitGTKOptions object. The driver constructor can receive an instance of a WebKitGTKOptions object, with the browser information and other options. Let’s see how it works with an example to launch epiphany:

from selenium import webdriver
from selenium.webdriver import WebKitGTKOptions

options = WebKitGTKOptions()
options.browser_executable_path = "/usr/bin/epiphany"
epiphany = webdriver.WebKitGTK(browser_options=options)

Again, this is just an example, Epiphany doesn’t even support WebDriver yet. Browsers or applications could create their own drivers on top of the WebKitGTK one to make it more convenient to use.

from selenium import webdriver
epiphany = webdriver.Epiphany()


During the next release cycle, we plan to do the following tasks:

  • Complete the implementation: add support for all commands in the spec and complete the ones that are partially supported now.
  • Add support for running the WPT WebDriver tests in the WebKit bots.
  • Add a WebKitGTK driver implementation for other languages in Selenium.
  • Add support for automation in Epiphany.
  • Add WebDriver support to WPE/dyz.

by carlos garcia campos at September 09, 2017 05:33 PM

September 06, 2017

Frédéric Wang

Review of Igalia's Web Platform activities (H1 2017)


For many years Igalia has been committed to and dedicated efforts to the improvement of Web Platform in all open-source Web Engines (Chromium, WebKit, Servo, Gecko) and JavaScript implementations (V8, SpiderMonkey, ChakraCore, JSC). We have been working in the implementation and standardization of some important technologies (CSS Grid/Flexbox, ECMAScript, WebRTC, WebVR, ARIA, MathML, etc). This blog post contains a review of these activities performed during the first half (and a bit more) of 2017.



A few years ago Bloomberg and Igalia started a collaboration to implement a new layout model for the Web Platform. Bloomberg had complex layout requirements and what the Web provided was not enough and caused performance issues. CSS Grid Layout seemed to be the right choice, a feature that would provide such complex designs with more flexibility than the currently available methods.

We’ve been implementing CSS Grid Layout in Blink and WebKit, initially behind some flags as an experimental feature. This year, after some coordination effort to ensure interoperability (talking to the different parties involved like browser vendors, the CSS Working Group and the web authors community), it has been shipped by default in Chrome 58 and Safari 10.1. This is a huge step for the layout on the web, and modern websites will benefit from this new model and enjoy all the features provided by CSS Grid Layout spec.

Since the CSS Grid Layout shared the same alignment properties as the CSS Flexible Box feature, a new spec has been defined to generalize alignment for all the layout models. We started implementing this new spec as part of our work on Grid, being Grid the first layout model supporting it.

Finally, we worked on other minor CSS features in Blink such as caret-color or :focus-within and also several interoperability issues related to Editing and Selection.


MathML is a W3C recommendation to represent mathematical formulae that has been included in many other standards such as ISO/IEC, HTML5, ebook and office formats. There are many tools available to handle it, including various assistive technologies as well as generators from the popular LaTeX typesetting system.

After the improvements we performed in WebKit’s MathML implementation, we have regularly been in contact with Google to see how we can implement MathML in Chromium. Early this year, we had several meetings with Google’s layout team to discuss this in further details. We agreed that MathML is an important feature to consider for users and that the right approach would be to rely on the new LayoutNG model currently being implemented. We created a prototype for a small LayoutNG-based MathML implementation as a proof-of-concept and as a basis for future technical discussions. We are going to follow-up on this after the end of Q3, once Chromium’s layout team has made more progress on LayoutNG.


Servo is Mozilla’s next-generation web content engine based on Rust, a language that guarantees memory safety. Servo relies on a Rust project called WebRender which replaces the typical rasterizer and compositor duo in the web browser stack. WebRender makes extensive use of GPU batching to achieve very exciting performance improvements in common web pages. Mozilla has decided to make WebRender part of the Quantum Render project.

We’ve had the opportunity to collaborate with Mozilla for a few years now, focusing on the graphics stack. Our work has focused on bringing full support for CSS stacking and clipping to WebRender, so that it will be available in both Servo and Gecko. This has involved creating a data structure similar to what WebKit calls the “scroll tree” in WebRender. The scroll tree divides the scene into independently scrolled elements, clipped elements, and various transformation spaces defined by CSS transforms. The tree allows WebRender to handle page interaction independently of page layout, allowing maximum performance and responsiveness.


WebRTC is a collection of communications protocols and APIs that enable real-time communication over peer-to-peer connections. Typical use cases include video conferencing, file transfer, chat, or desktop sharing. Igalia has been working on the WebRTC implementation in WebKit and this development is currently sponsored by Metrological.

This year we have continued the implementation effort in WebKit for the WebKitGTK and WebKit WPE ports, as well as the maintenance of two test servers for WebRTC: Ericsson’s p2p and Google’s apprtc. Finally, a lot of progress has been done to add support for Jitsi using the existing OpenWebRTC backend.

Since OpenWebRTC development is not an active project anymore and given libwebrtc is gaining traction in both Blink and the WebRTC implementation of WebKit for Apple software, we are taking the first steps to replace the original WebRTC implementation in WebKitGTK based on OpenWebRTC, with a new one based on libwebrtc. Hopefully, this way we will share more code between platforms and get more robust support of WebRTC for the end users. GStreamer integration in this new implementation is an issue we will have to study, as it’s not built in libwebrtc. libwebrtc offers many services, but not every WebRTC implementation uses all of them. This seems to be the case for the Apple WebRTC implementation, and it may become our case too if we need tighter integration with GStreamer or hardware decoding.


WebVR is an API that provides support for virtual reality devices in Web engines. Implementation and devices are currently actively developed by browser vendors and it looks like it is going to be a huge thing. Igalia has started to investigate on that topic to see how we can join that effort. This year, we have been in discussions with Mozilla, Google and Apple to see how we could help in the implementation of WebVR on Linux. We decided to start experimenting an implementation within WebKitGTK. We announced our intention on the webkit-dev mailing list and got encouraging feedback from Apple and the WebKit community.


ARIA defines a way to make Web content and Web applications more accessible to people with disabilities. Igalia strengthened its ongoing committment to the W3C: Joanmarie Diggs joined Richard Schwerdtfeger as a co-Chair of the W3C’s ARIA working group, and became editor of the Core Accessibility API Mappings, [Digital Publishing Accessibility API Mappings] (, and Accessible Name and Description: Computation and API Mappings specifications. Her main focus over the past six months has been to get ARIA 1.1 transitioned to Proposed Recommendation through a combination of implementation and bugfixing in WebKit and Gecko, creation of automated testing tools to verify platform accessibility API exposure in GNU/Linux and macOS, and working with fellow Working Group members to ensure the platform mappings stated in the various “AAM” specs are complete and accurate. We will provide more information about these activities after ARIA 1.1 and the related AAM specs are further along on their respective REC tracks.

Web Platform Predictability for WebKit

The AMP Project has recently sponsored Igalia to improve WebKit’s implementation of the Web platform. We have worked on many issues, the main ones being:

  • Frame sandboxing: Implementing sandbox values to allow trusted third-party resources to open unsandboxed popups or restrict unsafe operations of malicious ones.
  • Frame scrolling on iOS: Addressing issues with scrollable nodes; trying to move to a more standard and interoperable approach with scrollable iframes.
  • Root scroller: Finding a solution to the old interoperability issue about how to scroll the main frame; considering a new rootScroller API.

This project aligns with Web Platform Predictability which aims at making the Web more predictable for developers by improving interoperability, ensuring version compatibility and reducing footguns. It has been a good opportunity to collaborate with Google and Apple on improving the Web. You can find further details in this blog post.


Igalia has been involved in design, standardization and implementation of several JavaScript features in collaboration with Bloomberg and Mozilla.

In implementation, Bloomberg has been sponsoring implementation of modern JavaScript features in V8, SpiderMonkey, JSC and ChakraCore, in collaboration with the open source community:

  • Implementation of many ES6 features in V8, such as generators, destructuring binding and arrow functions
  • Async/await and async iterators and generators in V8 and some work in JSC
  • Optimizing SpiderMonkey generators
  • Ongoing implementation of BigInt in SpiderMonkey and class field declarations in JSC

On the design/standardization side, Igalia is active in TC39 and with Bloomberg’s support

In partnership with Mozilla, Igalia has been involved in the specification of various JavaScript standard library features for internationalization, in specification, implementation in V8, code reviews in other JavaScript engines, as well as working with the underlying ICU library.

Other activities

Preparation of Web Engines Hackfest 2017

Igalia has been organizing and hosting the Web Engines Hackfest since 2009. This event under an unconference format has been a great opportunity for Web Engines developers to meet, discuss and work together on the web platform and on web engines in general. We announced the 2017 edition and many developers already confirmed their attendance. We would like to thank our sponsors for supporting this event and we are looking forward to seeing you in October!

Coding Experience

Emilio Cobos has completed his coding experience program on implementation of web standards. He has been working in the implementation of “display: contents” in Blink but some work is pending due to unresolved CSS WG issues. He also started the corresponding work in WebKit but implementation is still very partial. It has been a pleasure to mentor a skilled hacker like Emilio and we wish him the best for his future projects!

New Igalians

During this semester we have been glad to welcome new igalians who will help us to pursue Web platform developments:

  • Daniel Ehrenberg joined Igalia in January. He is an active contributor to the V8 JavaScript engine and has been representing Igalia at the ECMAScript TC39 meetings.
  • Alicia Boya joined Igalia in March. She has experience in many areas of computing, including web development, computer graphics, networks, security, and software design with performance which we believe will be valuable for our Web platform activities.
  • Ms2ger joined Igalia in July. He is a well-known hacker of the Mozilla community and has wide experience in both Gecko and Servo. He has noticeably worked in DOM implementation and web platform test automation.


Igalia has been involved in a wide range of Web Platform technologies going from Javascript and layout engines to accessibility or multimedia features. Efforts have been made in all parts of the process:

  • Participation to standardization bodies (W3C, TC39).
  • Elaboration of conformance tests (web-platform-tests test262).
  • Implementation and bug fixes in all open source web engines.
  • Discussion with users, browser vendors and other companies.

Although, some of this work has been sponsored by Google or Mozilla, it is important to highlight how external companies (other than browser vendors) can make good contributions to the Web Platform, playing an important role on its evolution. Alan Stearns already pointed out the responsibility of the Web Plaform users on the evolution of CSS while Rachel Andrew emphasized how any company or web author can effectively contribute to the W3C in many ways.

As mentioned in this blog post, Bloomberg is an important contributor of several open source projects and they’ve been a key player in the development of CSS Grid Layout or Javascript. Similarly, Metrological’s support has been instrumental for the implementation of WebRTC in WebKit. We believe others could follow their examples and we are looking forward to seeing more companies sponsoring Web Platform developments!

September 06, 2017 10:00 PM

August 31, 2017

Xabier Rodríguez Calvar

Some rough numbers on WebKit code

My wife asked me for some rough LOC numbers on the WebKit project and I think I could share them with you here as well. They come from r221232. As I’ll take into account some generated code it is relevant to mention that I built WebKitGTK+ with the default CMake options.

First thing I did was running sloccount Source and got the following numbers:

cpp: 2526061 (70.57%)
ansic: 396906 (11.09%)
asm: 207284 (5.79%)
javascript: 175059 (4.89%)
java: 74458 (2.08%)
perl: 73331 (2.05%)
objc: 44422 (1.24%)
python: 38862 (1.09%)
cs: 13011 (0.36%)
ruby: 11605 (0.32%)
xml: 11396 (0.32%)
sh: 3747 (0.10%)
yacc: 2167 (0.06%)
lex: 1007 (0.03%)
lisp: 89 (0.00%)
php: 10 (0.00%)

This number do not include IDL code so I did some grepping to get the number myself that gave me 19632 IDL lines:

$ find Source/ -name ".idl" | xargs cat | grep -ve "^[[:space:]]\/*" -ve "^[[:space:]]*" -ve "^[[:space:]]$" -ve "^[[:space:]][$" -ve "^[[:space:]]};$" | wc -l

The interesting part of the IDL files is that they are used to generate code so those 19632 IDL lines expand to:

ansic: 699140 (65.25%)
cpp: 368720 (34.41%)
python: 1492 (0.14%)
xml: 1040 (0.10%)
javascript: 883 (0.08%)
asm: 169 (0.02%)
perl: 11 (0.00%)

Let’s have a look now at the LayoutTests (they test the functionality of WebCore + the platform). Tests are composed mainly by HTML files so if you run sloccount LayoutTests you get:

javascript: 401159 (76.74%)
python: 87231 (16.69%)
xml: 22978 (4.40%)
php: 4784 (0.92%)
ansic: 3661 (0.70%)
perl: 2726 (0.52%)
sh: 199 (0.04%)

It’s quite interesting to see that sloccount does not consider HTML which is quite relevant when you’re testing a web engine so again, we have to count them manually (thanks to Carlos López who helped me to properly grep here as some binary lines were giving me a headache to get the numbers):

find LayoutTests/ -name ".html" -print0 | xargs -0 cat | strings | grep -Pv "^[[:space:]]$" | wc -l

You can see 2205690 of “meaningful lines” that combine HTML + other languages that you can see above. I can’t substract here to just get the HTML lines because the number above take into account files with a different extension than HTML, though many of them do include other languages, specially JavaScript.

But the LayoutTests do not include only pure WebKit tests. There are some imported ones so it might be interesting to run the same procedure under LayoutTests/imported to see which ones are imported and not written directly into the WebKit project. I emphasize that because they can be written by WebKit developers in other repositories and actually I can present myself and Youenn Fablet as an example as we wrote tests some tests that were finally moved into the specification and included back later when imported. So again, sloccount LayoutTests/imported:

python: 84803 (59.99%)
javascript: 51794 (36.64%)
ansic: 3661 (2.59%)
php: 575 (0.41%)
xml: 250 (0.18%)
sh: 199 (0.14%)
perl: 86 (0.06%)

The same procedure to count HTML + other stuff lines inside that directory gives a number of 295490:

$ find LayoutTests/imported/ -name ".html" -print0 | xargs -0 cat | strings | grep -Pv "^[[:space:]]$" | wc -l

There are also some other tests that we can talk about, for example the JSTests. I’ll mention already the numbers summed up regarding languages and the manual HTML code (if you made it here, you know the drill already):

javascript: 1713200 (98.64%)
xml: 20665 (1.19%)
perl: 2449 (0.14%)
python: 421 (0.02%)
ruby: 56 (0.00%)
sh: 38 (0.00%)
HTML+stuff: 997


javascript: 297 (41.02%)
ansic: 187 (25.83%)
java: 118 (16.30%)
xml: 103 (14.23%)
php: 10 (1.38%)
perl: 9 (1.24%)
HTML+stuff: 16026


javascript: 950916 (83.12%)
cpp: 147194 (12.87%)
ansic: 38540 (3.37%)
asm: 5466 (0.48%)
sh: 872 (0.08%)
ruby: 419 (0.04%)
perl: 348 (0.03%)
python: 325 (0.03%)
xml: 5 (0.00%)
HTML+stuff: 238002


cpp: 44753 (99.45%)
ansic: 163 (0.36%)
objc: 76 (0.17%)
xml: 7 (0.02%)
javascript: 1 (0.00%)
HTML+stuff: 3887

And this is all. Remember that these are just some rough statistics, not a “scientific” paper.


In her expert opinion, in the WebKit project we are devoting around 50% of the total LOC to testing, which makes it a software engineering “textbook” project regarding testing and I think we can be proud of it!

by calvaris at August 31, 2017 09:03 AM

August 29, 2017

Frédéric Wang

The AMP Project and Igalia working together to improve WebKit and the Web Platform


The AMP Project and Igalia have recently been collaborating to improve WebKit’s implementation of the Web platform. Both teams are committed to make the Web better and we expect that all developers and users will benefit from this effort. In this blog post, we review some of the bug fixes and features currently being considered:

  • Frame sandboxing: Implementing sandbox values to allow trusted third-party resources to open unsandboxed popups or restrict unsafe operations of malicious ones.

  • Frame scrolling on iOS: Trying to move to a more standard and interoperable approach via iframe elements; addressing miscellaneous issues with scrollable nodes (e.g. visual artifacts while scrolling, view not scrolled when using “Find Text”…).

  • Root scroller: Finding a solution to the old interoperability issue about how to scroll the main frame; considering a new rootScroller API.

Some demo pages for frame sandboxing and scrolling are also available if you wish to test features discussed in this blog post.


AMP is an open-source project to enable websites and ads that are consistently fast, beautiful and high-performing across devices and distribution platforms. Several interoperability bugs and missing features in WebKit have caused problems to AMP users and to Web developers in general. Although it is possible to add platform-specific workarounds to AMP, the best way to help the Web Platform community is to directly fix these issues in WebKit, so that everybody can benefit from these improvements.

Igalia is a consulting company with a team dedicated to Web Platform developments in all open-source Web Engines (Chromium, WebKit, Servo, Gecko) working in the implementation and standardization of miscellaneous technologies (CSS Grid/flexbox, ECMAScript, WebRTC, WebVR, ARIA, MathML, etc). Given this expertise, the AMP Project sponsored Igalia so that they can lead these developments in WebKit. It is worth noting that this project aligns with the Web Predictability effort supported by both Google and Igalia, which aims at making the Web more predictable for developers. In particular, the following aspects are considered:

  • Interoperability: Effort is made to write Web Platform Tests (WPT), to follow Web standards and ensure consistent behaviors between web engines or operating systems.
  • Compatibility: Changes are carefully analyzed using telemetry techniques or user feedback in order to avoid breaking compatibility with previous versions of WebKit.
  • Reducing footguns: Removals of non-standard features (e.g. CSS vendor prefixes) are attempted while new features are carefully introduced.

Below we provide further description of the WebKit improvements, showing concretely how the above principles are followed.

Frame sandboxing

A sandbox attribute can be specified on the iframe element in order to enable a set of restrictions on any content it hosts. These conditions can be relaxed by specifying a list of values such as allow-scripts (to allow javascript execution in the frame) or allow-popups (to allow the frame to open popups). By default, the same restrictions apply to a popup opened by a sandboxed frame.

iframe sandboxing
Figure 1: Example of sandboxed frames (Can they navigate their top frame or open popups? Are such popups also sandboxed?)

However, sometimes this behavior is not wanted. Consider for example the case of an advertisement inside a sandboxed frame. If a popup is opened from this frame then it is likely that a non-sandboxed context is desired on the landing page. In order to handle this use case, a new allow-popups-to-escape-sandbox value has been introduced. The value is now supported in Safari Technology Preview 34.

While performing that work, it was noticed that some WPT tests for the sandbox attribute were still failing. It turns out that WebKit does not really follow the rules to allow navigation. More specifically, navigating a top context is never allowed when such context corresponds to an opened popup. We have made some changes to WebKit so that it behaves more closely to the specification. This is integrated into Safari Technology Preview 35 and you can for example try this W3C test. Note that this test requires to change preferences to allow popups.

It is worth noting that web engines may slightly depart from the specification regarding the previously mentioned rules. In particular, WebKit checks a same-origin condition to be sure that one frame is allowed to navigate another one. WebKit always has contained a special case to ignore this condition when a sandboxed frame with the allow-top-navigation flag tries and navigate its top frame. This feature, sometimes known as “frame busting,” has been used by third-party resources to perform malicious auto-redirecting. As a consequence, Chromium developers proposed to restrict frame busting to the case where the navigation is triggered by a user gesture.

According to Chromium’s telemetry frame busting without a user gesture is very rare. But when experimenting with the behavior change of allow-top-navigation several regressions were reported. Hence it was instead decided to introduce the allow-top-navigation-by-user-activation flag in order to provide this improved safety context while still preserving backward compatibility. We implemented this feature in WebKit and it is now available in Safari Technology Preview 37.

Finally, another proposed security improvement is to use an allow-modals flag to explicitly allow sandboxed frames to display modal dialogs (with alert, prompt, etc). That is, the default behavior for sandboxed frames will be to forbid such modal dialogs. Again, such a change of behavior must be done with care. Experiments in Chromium showed that the usage of modal dialogs in sandboxed frames is very low and no users complained. Hence we implemented that behavior in WebKit and the feature should arrive in Safari Technology Preview soon.

Check out the frame sandboxing demos if if you want to test the new allow-popup-to-escape-sandbox, allow-top-navigation-without-user-activation and allow-modals flags.

Frame scrolling on iOS

Apple’s UI choice was to (almost) always “flatten” (expand) frames so that users do not require to scroll them. The rationale for this is that it avoids to be trapped into hierarchy of nested frames. Changing that behavior is likely to cause a big backward compatibility issue on iOS so for now we proposed a less radical solution: Add a heuristic to support the case of “fullscreen” iframes used by the AMP Project. Note that such exceptions already exist in WebKit, e.g. to avoid making offscreen content visible.

We thus added the following heuristic into WebKit Nightly: do not flatten out-of-flow iframes (e.g. position: absolute) that have viewport units (e.g. vw and vh). This includes the case of the “fullscreen” iframe previously mentioned. For now it is still under a developer flag so that WebKit developers can control when they want to enable it. Of course, if this is successful we might consider more advanced heuristics.

The fact that frames are never scrollable in iOS is an obvious interoperability issue. As a workaround, it is possible to emulate such “scrollable nodes” behavior using overflow: scroll nodes with the -webkit-overflow-scrolling: touch property set. This is not really ideal for our Web Predictability goal as we would like to get rid of browser vendor prefixes. Also, in practice such workarounds lead to even more problems in AMP as explained in these blog posts. That’s why implementing scrolling of frames is one of the main goals of this project and significant steps have already been made in that direction.

Class Hierarchy
Figure 2: C++ classes involved in frame scrolling

The (relatively complex) class hierarchy involved in frame scrolling is summarized in Figure 2. The frame flattening heuristic mentioned above is handled in the WebCore::RenderIFrame class (in purple). The WebCore::ScrollingTreeFrameScrollingNodeIOS and WebCore::ScrollingTreeOverflowScrollingNodeIOS classes from the scrolling tree (in blue) are used to scroll, respectively, the main frame and overflow nodes on iOS. Scrolling of non-main frames will obviously have some code to share with the former, but it will also have some parts in common with the latter. For example, passing an extra UIScrollView layer is needed instead of relying on the one contained in the WKWebView of the main frame. An important step is thus to introduce a special class for scrolling inner frames that would share some logic from the two other classes and some refactoring to ensure optimal code reuse. Similar refactoring has been done for scrolling node states (in red) to move the scrolling layer parameter into WebCore::ScrollingStateNode instead of having separate members for WebCore::ScrollingStateOverflowScrollingNode and WebCore::ScrollingStateFrameScrollingNode.

The scrolling coordinator classes (in green) are also important, for example to handle hit testing. At the moment, this is not really implemented for overflow nodes but it might be important to have it for scrollable frames. Again, one sees that some logic is shared for asynchronous scrolling on macOS (WebCore::ScrollingCoordinatorMac) and iOS (WebCore::ScrollingCoordinatorIOS) in ancestor classes. Indeed, our effort to make frames scrollable on iOS is also opening the possibility of asynchronous scrolling of frames on macOS, something that is currently not implemented.

Class Hierarchy
Figure 4: Video of this demo page on WebKit iOS with experimental patches to make frame scrollables (2017/07/10)

Finally, some more work is necessary in the render classes (purple) to ensure that the layer hierarchies are correctly built. Patches have been uploaded and you can view the result on the video of Figure 4. Notice that this work has not been reviewed yet and there are known bugs, for example with overlapping elements (hit testing not implemented) or position: fixed elements.

Various other scrolling bugs were reported, analyzed and sometimes fixed by Apple. The switch from overflow nodes to scrollable iframes is unlikely to address them. For example, the “Find Text” operation in iOS has advanced features done by the UI process (highlight, smart magnification) but the scrolling operation needed only works for the main frame. It looks like this could be fixed by unifying a bit the scrolling code path with macOS. There are also several jump and flickering bugs with position: fixed nodes. Finally, Apple fixed inconsistent scrolling inertia used for the main frame and the one used for inner scrollable nodes by making the former the same as the latter.

Root Scroller

The CSSOM View specification extends the DOM element with some scrolling properties. That specification indicates that the element to consider to scroll the main view is document.body in quirks mode while it is document.documentElement in no-quirks mode. This is the behavior that has always been followed by browsers like Firefox or Interner Explorer. However, WebKit-based browsers always treat document.body as the root scroller. This interoperability issue has been a big problem for web developers. One convenient workaround was to introduce the document.scrollingElement which returns the element to use for scrolling the main view (document.body or document.documentElement) and was recently implemented in WebKit. Use this test page to verify whether your browser supports the document.scrollingElement property and which DOM element is used to scroll the main view in no-quirks mode.

Nevertheless, this does not solve the issue with existing web pages. Chromium’s Web Platform Predictability team has made a huge communication effort with Web authors and developers which has drastically reduced the use of document.body in no-quirks mode. For instance, Chromium’s telemetry on Figure 3 indicates that the percentage of document.body.scrollTop in no-quirks pages has gone from 18% down to 0.0003% during the past three years. Hence the Chromium team is now considering shipping the standard behavior.

UseCounter for ScrollTopBodyNotQuirksMode
Figure 3: Use of document.body.scrollTop in no-quirks mode over time (Chromium's UseCounter)

In WebKit, the issue has been known for a long time and an old attempt to fix it was reverted for causing regressions. For now, we imported the CSSOM View tests and just marked the one related to the scrolling element as failing. An analysis of the situation has been left on WebKit’s bug; Depending on how things evolve on Chromium’s side we could consider the discussion and implementation work in WebKit.

Related to that work, a new API is being proposed to set the root scroller to an arbitrary scrolling element, giving more flexibility to authors of Web applications. Today, this is unfortunately not possible without losing some of the special features of the main view (e.g. on iOS, Safari’s URL bar is hidden when scrolling the main view to maximize the screen space). Such API is currently being experimented in Chromium and we plan to investigate whether this can be implemented in WebKit too.


In the past months, The AMP Project and Igalia have worked on analyzing some interoperability issue and fixing them in WebKit. Many improvements for frame sandboxing are going to be available soon. Significant progress has also been made for frame scrolling on iOS and collaboration continues with Apple reviewers to ensure that the work will be integrated in future versions of WebKit. Improvements to “root scrolling” are also being considered although they are pending on the evolution of the issues on Chromium’s side. All these efforts are expected to be useful for WebKit users and the Web platform in general.

Igalia Logo
AMP Logo

Last but not least, I would like to thank Apple engineers Simon Fraser, Chris Dumez, and Youenn Fablet for their reviews and help, as well as Google and the AMP team for supporting that project.

August 29, 2017 10:00 PM

August 09, 2017

Michael Catanzaro

On Firefox Sync

Epiphany 3.26 is, unfortunately, not going to be packed with cool new features like 3.24 was. We’ve just been too busy working on improving WebKit this cycle. But there is one cool new thing: Firefox Sync support. You can sync bookmarks, history, passwords, and open tabs with other Epiphany instances and as well as both desktop and mobile Firefox. This is already enabled in 3.25.90. Just go to the Sync tab in Preferences and sign in or create your Firefox account there. Please test it out and report bugs now, so we can quash problems you find before 3.26.0 rather than after.

Some thank yous are in order:

  • Thanks to Gabriel Ivascu, for writing all the code.
  • Thanks to Google and Igalia for sponsoring Gabriel’s work.
  • Thanks to Mozilla. This project would never have been possible if Mozilla had not carefully written its terms of service to allow such use.

Go forth and sync!

by Michael Catanzaro at August 09, 2017 07:57 PM

August 06, 2017

Michael Catanzaro

Endgame for WebKit Woes

In my original blog post On WebKit Security Updates, I identified three separate problems affecting WebKit users on Linux:

  • Distributions were not providing updates for WebKitGTK+. This was the main focus of that post.
  • Distributions were shipping a insecure compatibility package for old, unmaintained WebKitGTK+ 2.4 (“WebKit1”).
  • Distributions were shipping QtWebKit, which was also unmaintained and insecure.

Let’s review these problems one at a time.

Distributions Are Updating WebKitGTK+

Nowadays, most major community distributions are providing regular WebKitGTK+ updates, so this is no longer a problem for the vast majority of Linux users. If you’re using a supported version of Ubuntu (except Ubuntu 14.04), Fedora, or most other mainstream distributions, then you are good to go.

My main concern here is still Debian, but there are reasons to be optimistic. It’s too soon to say what Debian’s policy will be going forward, but I am encouraged that it broke freeze just before the Stretch release to update from WebKitGTK+ 2.14 to 2.16.3. Debian is slow and conservative and so has not yet updated to 2.16.6, which is sad because 2.16.3 is affected by a bug that causes crashes on a huge number of websites, but my understanding is it is likely to be updated in the near future. I’m not sure if Debian will update to 2.18 or not. We’ll have to wait and see.

openSUSE is another holdout. The latest stable version of openSUSE Leap, 42.3, is currently shipping WebKitGTK+ 2.12.5. That is disappointing.

Most other major distributions seem to be current.

Distributions Are Removing WebKitGTK+ 2.4

WebKitGTK+ 2.4 (often informally referred to as “WebKit1”) was the next problem. Tons of desktop applications depended on this old, insecure version of WebKitGTK+, and due to large API changes, upgrading applications was not going to be easy. But this transition is going much smoother and much faster than I expected. Several distributions, including Debian, Fedora, and Arch, have recently removed their compatibility packages. There will be no WebKitGTK+ 2.4 in Debian 10 (Buster) or Fedora 27 (scheduled for release this October). Most noteworthy applications have either ported to modern WebKitGTK+, or have configure flags to disable use of WebKitGTK+. In some cases, such as GnuCash in Fedora, WebKitGTK+ 2.4 is being bundled as part of the application build process. But more often, applications that have not yet ported simply no longer work or have been removed from these distributions.

Soon, users will no longer need to worry that a huge amount of WebKitGTK+ applications are not receiving security updates. That leaves one more problem….

QtWebKit is Back

Upstream QtWebKit has not been receiving security updates for the past four years or thereabouts, since it was abandoned by the Qt project. That is still the status quo for most distributions, but Arch and Fedora have recently switched to Konstantin Tokarev’s fork of QtWebKit, which is based on WebKitGTK+ 2.12. (Thank you Konstantin!) If you are using any supported version of Fedora, you should already have been switched to this fork. I am hopeful that the fork will be rebased on WebKitGTK+ 2.16 or 2.18 in the near future, to bring it current on security updates, but in the meantime, being a year and a half behind is an awful lot better than being four years behind. Now that Arch and Fedora have led the way, other distributions should find little trouble in making the switch to Konstantin’s QtWebKit. It would be a disservice to users to continue shipping the upstream version.

So That’s Cool

Things are better. Some distributions, notably Arch and Fedora, have resolved all of the above problems (or will in the very near future). Yay!

by Michael Catanzaro at August 06, 2017 09:47 PM

July 04, 2017

Charlie Turner

Qt Creator Tips for WebKit GTK


Using an IDE on GNU/Linux for C/C++ development is slightly contentious in many circles. People either seem to not find IDE’s value-add worthwhile compared to their cult text editor + UNIX tools, or have tried them in the past and not had good experiences, so soilder on with the cult text editor approach. I’ve tended to be in the latter camp of people, knowing that in a perfect world an IDE would help me, but they don’t seem to be up-to-snuff in this environment yet.

I’d limped along with taggers like GNU global and etags alongside Emacs. Cortored find+grep commands wired into Emacs’ helm package were my “Find all references” and “Jump to definition”. It worked to an extent, but it does feel a little primitive, and GUD always frustrated me. New semantic taggers such as SourceWeb and rtags looked interesting and hope they continue to mature, but I was struggling getting WebKit through them. The Clang tooling is also rather slow at processing the source files, upon which both these tools are based.

You can make Qt Creator build and install WebKit into a jhbuild for, say, Epiphany. I describe those steps if you’re inclined have that full IDE experience. The instructions below are annotated with [Building?] for steps that are applicable only to that configuration. I don’t personally do this because I prefer to run the build/install commands outside the IDE. With those introductions out of the way, what I’ve ended up with is a decent code navigator (alternative to Eclipse!) and a good debugger frontend. I’m happier with the combination of cult editor + Qt Creator for working on C++ projects. It’s not perfect, but I hope you might find it useful as well.

First get Qt Creator installed. If using your distributions package manager, just check the version is fairly recent. If it isn’t, download if from the Qt Creator site, but during the installation process, I recommend not installing the Qt libraries.

Load the project into Qt Creator

  • Always run Qt Creator from the WebKit jhbuild environment. E.g., ./Tools/jhbuild/jhbuild-wrapper --gtk run /usr/bin/qtcreator. If you don’t, CMake will find all kinds of random junk it calls dependencies on your system, if you’re lucky.
  • Go to File > Open File or Project.
  • Navigate to the top-level CMakeLists.txt in the WebKit checkout.
  • In the Configure Project screen, change the build directory output to taste. If you’re not planning on building from IDE, this doesn’t matter.
  • [Building?] For build directories, you could put it in $WEBKIT_ROOT/WebKitBuild/{Release,Debug} to match WebKit conventions. Don’t bother with the other configurations, especially not RelWithDebInfo, there are problems in WebKit with this configuration. Now click on Manage in the Desktop kit page (it’s one of those buttons that magically appears on the screen when you hover over it…), scroll down to CMake Configuration and click Change. Remove the Qt install prefix definitions, and add CMAKE_INSTALL_PREFIX:INTERNAL=/path/to/jhbuild/install/ and CMAKE_INSTALL_LIBDIR:INTERNAL=lib. Note carefully that shell variables are not expanded here, so don’t use something like $HOME. You can also change the compiler and debugger used by the kit as well. Also make sure Qt version is None.
  • Once you get into the IDE after these steps, CMake will fail because we haven’t specified a port.
  • From the mode selector, click Projects (thing with the wrench icon) and go to the build settings. Set PORT=GTK and also add a boolean property for DEVELOPER_MODE=ON and ENABLE_WAYLAND_TARGET=OFF if you’re working on X11.
  • [Building?] Click Advanced if you’d like to change the default compiler switches for Debug/Release configurations. With GCC I like to use -ggdb -Og in Debug.
  • Configuring should now suceed
  • [Building?] Click Build and it will likely fail. I’ve found Qt Creator needs to be restarted at this point. Restart and the build should now work.
  • [Building?] As a finishing touch, you can configure the run configuration for launching Epiphany. In the Run Settings window under the projects mode, create a custom deploy step to run command jhbuild with arguments run cmake -P cmake_install.cmake. This will install WebKit in the jhbuild environment. Now add a custom executable and specify the executable to be jhbuild and the arguments to be run epiphany. The Run button will now install WebKit for use by Epiphany and launch the browser ready for attachment (see next section).


  • Due to the multi-process nature of WebKit, you can’t just click on “Start Debugging”, since there’s several processes you might want to attach to. Launch WebKit and once it’s running, go to Debug > Start Debugging > Attach to Running Application and select the PID of the process you’d like to attach to.
  • It’s likely Qt Creator will time out the GDB launch and ask if you’d like to give it more time. Say yes, and go to Tools > Options > Debugger > GDB and bump the timeout up to 60 seconds.
  • If you’re getting assembly instructions when you hit a breakpoint, it’s likely your source isn’t getting found with the debugger. This shouldn’t happen to you, but if it does you’ll want to add ../../Source -> $WEBKIT_CHECKOUT/Source source mapping. This can be done in Tools > Options > Debugger > General. The build system doesn’t force
    the compiler to emit absolute paths in debugging info (there are ways around that, but this is easier)
  • GDB commands can be issued by bringing up the poorly named “Debugger Log” in the debugger views menu. Some helpful commands I’ve used on WebKit are handle SIGUSR1 noprint to stop being interrupted by IPC, and set scheduler-locking on to single-step through the current thread (you really don’t want to enable that from the start though 😉 just use it in the middle of debug session when you want to step a thread).
  • Everything else I’ve found convenient to do via the IDE.


  • Header files don’t have their #if parsed properly, I think because the config.h is indirectly available to header files, which is really unfriendly to static analysis tools used by IDEs. This is with the default code model, I’m sure it would be better if you try the Clang code model, but the current support for that in Qt Creator is limited, and the tradeoff is much, much slower indexing. This isn’t really an issue with the IDE but rather the coding style guidelines of WebKit.
  • Switching kits often requires restarting the IDE, otherwise you get build step errors. I’m guessing this has something to do with the CMake caching the IDE uses. When in doubt, restart the IDE.
  • When you do an expensive interaction with the code model, it blocks the UI thread rendering the whole IDE unresponsive. This is much worse with the Clang code model because it’s so much slower than the default. Can be a problem with the Qt Code Model if you ask for things like the type hierachy.

by cturner at July 04, 2017 12:58 PM

June 15, 2017

Michael Catanzaro

Debian Stretch ships latest WebKitGTK+

I’ll keep this update short. Debian has decided to ship the latest version of WebKitGTK+, 2.16.3, in its upcoming Stretch release. Since Debian was the last major distribution holding out on providing WebKit security updates, this is a big deal. Huge thanks to Jeremy Bicha for making this possible.

The bad news is that Debian is still considering whether or not to provide periodic security updates after the release, so there might not be any. But maybe there will be. We will have to wait and see. At least releasing with the latest version is a big step in the right direction.

by Michael Catanzaro at June 15, 2017 04:34 PM