The primary goals of this change are:
- Better conformance to the W3C flexbox standard (https://www.w3.org/TR/css-flexbox-1/)
and a clear articulation of the areas where it deviates from the spec.
- Support for flex-shrink.
- Conformance with layout effects of "overflow: hidden".
Specifically, here are the limitations of this implementation as compared to the W3C
flexbox standard (this is also documented in Layout.js):
- Display property is always assumed to be 'flex' except for Text nodes, which
are assumed to be 'inline-flex'.
- The 'zIndex' property (or any form of z ordering) is not supported. Nodes are
stacked in document order.
- The 'order' property is not supported. The order of flex items is always defined
by document order.
- The 'visibility' property is always assumed to be 'visible'. Values of 'collapse'
and 'hidden' are not supported.
- The 'wrap' property supports only 'nowrap' (which is the default) or 'wrap'. The
rarely-used 'wrap-reverse' is not supported.
- Rather than allowing arbitrary combinations of flexGrow, flexShrink and
flexBasis, this algorithm supports only the three most common combinations:
- flex: 0 is equiavlent to flex: 0 0 auto
- flex: n (where n is a positive value) is equivalent to flex: n 0 0
- flex: -1 (or any negative value) is equivalent to flex: 0 1 auto
- Margins cannot be specified as 'auto'. They must be specified in terms of pixel
values, and the default value is 0.
- The 'baseline' value is not supported for alignItems and alignSelf properties.
- Values of width, maxWidth, minWidth, height, maxHeight and minHeight must be
specified as pixel values, not as percentages.
- There is no support for calculation of dimensions based on intrinsic aspect ratios
(e.g. images).
- There is no support for forced breaks.
- It does not support vertical inline directions (top-to-bottom or bottom-to-top text).
And here is how the implementation deviates from the standard (this is also documented in
Layout.js):
- Section 4.5 of the spec indicates that all flex items have a default minimum
main size. For text blocks, for example, this is the width of the widest word.
Calculating the minimum width is expensive, so we forego it and assume a default
minimum main size of 0.
- Min/Max sizes in the main axis are not honored when resolving flexible lengths.
- The spec indicates that the default value for 'flexDirection' is 'row', but
the algorithm below assumes a default of 'column'.
