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.
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.
Using flex layout inside button solves the issue with wrongly calculated
height when it has: pseudo element and whitespaces inside.
Also using flex instead of a table layout allows for the same vertical
alignment but with fewer layout nodes: a flex container and anonymous
wrapper for content instead of a table wrapper, table, row, and cell.
