Add support for tracking containing blocks when doing inline layout.
This requires setting up a PositioningContext for inline boxes when
necessary. Instead of using the PositioningContext helper methods
and we reuse the contexts between line breaks.
Fixes#25279.
Add a few helper methods which allow removing duplicate code in
PositioningContext. These methods will also be used to properly
implement hoisting in inline layout.
Instead of painting hoisted position fragments in the order to which
they are hoisted, paint them in tree order and properly incorporate them
into the stacking context.
We do this by creating a placeholder fragment in the original tree position
of hoisted fragments. The ghost fragment contains an atomic id which
links back to the hoisted fragment in the containing block.
While building the stacking context, we keep track of containing blocks
and their children. When encountering a placeholder fragment we look at
the containing block's hoisted children in order to properly paint the
hoisted fragment.
One notable design modification in this change is that hoisted fragments
no longer need an AnonymousFragment as their parent. Instead they are
now direct children of the fragment that establishes their containing block.
This is a feature that was never properly implemented in the previous
layout system. We still need to preserve their in-tree order in the
display list though.
This method doesn't actually do any layout, but converts this block to a
HoistedAbsolutelyPositionedBox which is hoisted and then laid-out with
its containing block later. This makes the code a little easier to read.
Iits details are now private to the module.
It has a couple methods that take closures to make sure that "before" and "after" steps are done together:
* In an absolutely positioned box, take care of nested abspos (establish a new containing block, etc.)
* For a box that *might* be `position: relative`, optionally take care of the same.
Percentage `width` are treated as `auto` for the purpose of
min/max-content computation, so they also need to be considered
when testing “wether width is auto”
… and has a private enum for its contents.
Privacy forces the rest of the code to go through methods
rather than matching on the enum,
reducing accidental layout-mode-specific behavior.
