ALL THE PATCHES HAVE BEEN UPSTREAMED FROM MARCH TO AUGUST 2016. THE REFACTORING IS NOW COMPLETE
Overview
For the rationale and initial plan see Alex's message on webkit-dev mailing list.
The idea is now to remove the flexbox dependency (CSS properties & class inheritance) only after having rewritten MathML layout functions from scratch. Hence we no longer need to temporarily "duplicate" flexbox code and this should make review easier. When refactoring renderer classes, we also remove the anonymous renderers (and their associated style) that were created for the sake of the flexbox-based layout.
Layout changes
The general box model for MathML renderers is available on the MathML in HTML5 note where you can also find other rules specific to each element. Note that these rules strongly relies on TeX / OpenType MATH and so math rendering requires appropriate fonts to look good. During this first step, we do not always follow completely the layout suggested in that note. Also for now, we ignore the distinction between ink VS logical metrics as well as classical CSS features like margin, padding, border, absolute position, vertical layout or linebreaking. However we do pay attention to right-to-left rendering of mathematical formulas.
There are essentially four functions to look at:
- computePreferredLogicalWidths: it sets the preferred min/max width. For now we do not do math line breaking so these widths are equal. They are only an estimate of the final width, because we do not know the exact width of stretchy operators (e.g. vertical parenthesis) until they get their final size.
- layoutBlock: it calculates the (exact) logical width, the logical height and and position of child renderers.
- firstLineBaseLine: it calculates the ascent of the renderer.
- paint: it does special painting for e.g. fraction line or root overbar.
Phase 1
The initial goal is 10) below. It now depends on the following bugs. Patches are generally more or less independent, although test expectations will depend on the exact order in which they are applied.
1) RenderMathMLRow: layout a row of math items (numbers, operators, variables... or any subformulas), maybe with some operators stretching vertically.
2) RenderMathMLUnderOver: base with some scripts above and below.
3) RenderMathMLFraction: layout a fraction with numerator & denominator. The patch also fixes some bugs and uses some parameters from the OpenType MATH table.
4) RenderMathMLScripts: base with arbitrary number of pre/post scripts. Note that the patch introduces many parameters from the OpenType MATH table so be sure to read the corresponding section in the MathML in HTML5 note.
5) MathOperator module: Some stretchy operators (e.g. the radical sign) are currently drawn using anonymous RenderMathMLOperators. Since we want to get rid of anonymous renderers, we need this preliminary step. It also improves a bit the rendering of horizontal operators. See
bug 156538,
bug 156542,
bug 156571,
bug 156572,
bug 156574,
bug 156910,
bug 156913,
bug 156921,
bug 156950,
bug 157071 and finally
bug 152244
6) RenderMathMLRoot: square root or arbitrary N-th root. See bug 156836 and bug 153987.
7) RenderMathMLOperator and RenderMathMLToken: Simple text (operators, variables, numbers etc). This removes anonymous renderers but the layout functions still rely on the RenderFlexibleBox's implementation. A new generic layout code will only be introduced for RenderMathMLBlock in bug 153991. See
bug 157519,
bug 157521,
bug 157568
and finally bug 155018.
8) RenderMathMLMenclose: A RenderMathMLRow with some additional notations on it (strikes, boxes, circles etc). It is the follow up of bug 153208. See also this proposal.
9) RenderMathMLSpace: Simple renderer to insert blank boxes for adjusting spacing of math formulas.
10) Remove flexbox dependency: This is the final patch to make RenderMathMLBlock inherit from RenderBlock rather than RenderFlexibleBox
Phase 2
In a second phase we add support for important MathML features and OpenType MATH properties:
11) Implement an internal style property for displaystyle
12) Add support for movablelimits
13) Add support for mathvariants that cannot be emulated via CSS
14) Use OpenType MATH constant AxisHeight
15) Use Radical* constants from the OpenType MATH table
16) Use Fraction* parameters from the OpenType MATH table
17) Use Stack* parameters from the OpenType MATH table
18) Use parameters from the OpenType MATH table for <munderover>
19) Basic implementation of mpadded
20) Add support for @href attribute in MathML
21) Use OpenType MATH fonts by default. See bug 133603 and bug 156840
Phase 3
In this third phase, we try and move all parsing of MathML attributes from renderer classes to element classes. Hopefully, this will also improve how we handle the update of the renderers when dynamic changes of attributes happen.
22) Create a MathMLLength struct to handle the parsing of MathML length
23) Move parsing of mspace attributes to a MathMLSpaceElement class
24) Move parsing of mpadded attributes to a MathMLPaddedElement class
25) Move parsing of subscriptshift and superscriptshift from rendering to element classes
26) Move parsing of mfrac attributes into a MathMLFractionElementClass
27) Move parsing of accentunder and accent attributes from renderer to element classes
29) Move parsing of operator attributes into the MathML element classes. See also
bug 159619,
bug 160034
bug 160241,
bug 160245,
bug 160400,
bug 160239,
bug 160190,
bug 160301,
160336 and
160339.
Accessibility Changes
The following patches impact the a11y code:
3) RenderMathMLFraction: The Mac accessibility code seems to need the linethickness relative to the default linethickness (which now depends on the math font).
4) RenderMathMLScripts: Some small box shifts happen.
6) RenderMathMLRoot: Anonymous wrappers are removed.
7) RenderMathMLOperator and RenderMathMLToken: The text content of token elements is now properly exposed into the accessibility tree. The management of anonymous child operators of the mfenced element must be adapted. See bug 139582
