This is a part of refactoring towards making the paintable tree
independent of the layout tree. Now, instead of transferring text
fragments from the layout tree to the paintable tree during the layout
commit phase, we allocate separate PaintableFragments that contain only
the information necessary for painting. Doing this also allows us to
get rid LineBoxes, as they are used only during layout.
This patch makes a few changes to the way we calculate line-height:
- `line-height: normal` is now resolved using metrics from the used
font (specifically, round(A + D + lineGap)).
- `line-height: calc(...)` is now resolved at style compute time.
- `line-height` values are now absolutized at style compute time.
As a consequence of the above, we no longer need to walk the DOM
ancestor chain looking for line-heights during style computation.
Instead, values are inherited, resolved and absolutized locally.
This is not only much faster, but also makes our line-height metrics
match those of other engines like Gecko and Blink.
When a button should use flex for alignment and also has ::before
and/or ::after, we previously did the following:
1. Prepended/appended the button's children with boxes for
pseudo-elements.
2. Replaced the button's direct children with a flex container that
contains its children.
As a result, the generated boxes for ::before/::after ended up as
children of the generated flex item, instead of being direct children
of the button layout box as they were supposed to be.
This change reverses these steps, ensuring that boxes for
pseudo-elements are generated only after modifications inside the
button layout are completed.
