[webkit-dev] MathML layout refactor proposal

[Zalan Bujtas]

It’s great to hear that MathML is getting some proper cleanup. I think the proposal sounds good and I am looking forward to the patches. 

Minor observation: I noticed that, since MathML block is no longer a flexbox, but it still needs some of the flexbox functionality, the following pattern has been introduced to the generic renderbox/block code.     
-    if (parent()->isFlexibleBox()) {
+    if (parent()->isFlexibleBox() || parent()->isRenderMathMLBlock()) {
I just wish there was another way to handle this.

Zalan.

> On Dec 9, 2015, at 3:35 AM, Alejandro Garcia Castro <alex at igalia.com> wrote:
> 
> 
> Hi,
> 
> In the last months we have been working on refactoring the MathML
> layout code to make it more maintainable, because in the current point
> fixing issues or adding new features has been too complex.
> 
> We have reached the point where we think we have to discuss whether
> this work makes sense and whether it is interesting for the WebKit
> community.  If so, we will start pushing the initial patches from our
> branch to upstream.
> 
> The idea behind the refactor is to remove FlexBox dependency and
> create its own layout MathML methods. The main reasons to do this are:
> 
>   - Reduce code complexity: Adapting FlexBox layout (which is already
>     complex) in order to create the MahtML layout made the code too
>     complex, a big technical debt that made the improvement of MathML
>     more difficult every day.
> 
>   - Avoid just another FlexBox dependent code: When we had to add
>     general layout alignment support we had a lot of problems trying
>     to solve some the MathML issues because it was not clear how it
>     worked with the FlexBox.
> 
>   - Improve performance: We do not need all the features FlexBox
>     layout adds for most of the MathML blocks but we are executing
>     all that code. We also can simplify the render tree structures
>     that were created to make the FlexBox layout work.
> 
>   - Make easier to improve the MathML implementation: Using
>     independent renderer classes gives more flexibility to get exact
>     positioning and spacing required to get high-quality math
>     rendering based on TeX rules and the OpenType MATH table.  (cf
>     http://www.mathml-association.org/MathMLinHTML5/)
> 
> We have a working prototype that basically passes the current MathML
> tests and removes the FlexBox dependency in:
> 
> https://github.com/alexgcastro/webkit/tree/MathMLLayout
> 
> We have basically one commit per MathML renderer that we had to
> replace. This is still initial code and we need more work and add more
> tests to make sure we are improving the situation.
> 
> We want to do it incrementally with 2 steps:
> 1. Remove FlexBox dependency but do not break the tests, this means
>   keep the RenderTree structure. This is basically done, and we just
>   need review and try to push them.
> 2. Refactor the RenderTree structure, removing the anonymous nodes
>   created to make FlexBox work and all the code that it was
>   required. We already have the Fractions and Underover implemented
>   and the code is much clearer. This will break the tests relying on
>   a PNG image or a render tree reference. Hence we will do it also
>   per renderer and rebasing the tests after each commit.
> 
> The main con of the change that the code could be bigger in some parts
> of the renderers, but more direct and simple, so it should be actually
> good regarding maintenance.
> 
> If you have any question, proposal or comment just send it, it would
> be great to hear some more feedback, and check if we should proceed
> with this effort.
> 
> Greetings,
> 
> Alex
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