ALL THE PATCHES HAVE BEEN UPSTREAMED FROM MARCH TO AUGUST 2016. THE REFACTORING IS NOW COMPLETE
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.
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.
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 157071 and finally
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
and finally bug 155018.
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
In a second phase we add support for important MathML features and OpenType MATH properties:
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.
29) Move parsing of operator attributes into the MathML element classes. See also
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