Since we always pass the px value as an argument to resolved(), we can
pass it directly as CSSPixels instead of wrapping it in Length. This
approach allows us to avoid converting to a double, resulting in fewer
precision issues.
This is intended to annotate conversions from unknown floating-point
values to CSSPixels, and make it more obvious the fp value will be
rounded to the nearest fixed-point value.
In general it is not safe to convert any arbitrary floating-point value
to CSSPixels. CSSPixels has a resolution of 0.015625, which for small
values (e.g. scale factors between 0 and 1), can produce bad results
if converted to CSSPixels then scaled back up. In the worst case values
can underflow to zero and produce incorrect results.
Although DistinctNumeric, which is supposed to abstract the underlying
type, was used to represent CSSPixels, we have a whole bunch of places
in the layout code that assume CSSPixels::value() returns a
floating-point type. This assumption makes it difficult to replace the
underlying type in CSSPixels with a non-floating type.
To make it easier to transition CSSPixels to fixed-point math, one step
we can take is to prevent access to the underlying type using value()
and instead use explicit conversions with the to_float(), to_double(),
and to_int() methods.
This fixes a plethora of rounding problems on many websites.
In the future, we may want to replace this with fixed-point arithmetic
(bug #18566) for performance (and consistency with other engines),
but in the meantime this makes the web look a bit better. :^)
There's a lot more things that could be converted to doubles, which
would reduce the amount of casting necessary in this patch.
We can do that incrementally, however.
VALUES-4 defines the internal representation of `calc()` as a tree of
calculation nodes. ( https://www.w3.org/TR/css-values-4/#calc-internal )
VALUES-3 lacked any definition here, so we had our own ad-hoc
implementation based around the spec grammar. This commit replaces that
with CalculationNodes representing each possible node in the tree.
There are no intended functional changes, though we do now support
nested calc() which previously did not work. For example:
`width: calc( 42 * calc(3 + 7) );`
I have added an example of this to our test page.
A couple of the layout tests that used `calc()` now return values that
are 0.5px different from before. There's no visual difference, so I
have updated the tests to use the new results.
Level 4 drops the limitations of what types can be a denominator, which
means `<calc-number-sum>`, `<calc-number-product>` and
`<calc-number-value>` all go away.
