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.
Before this change, we were doing it after every layout, which meant
that already-propagated overflow could be propagated again, which led to
incorrect scrolling behavior.
Some replaced elements can have intrinsic aspect ratios but no
intrinsic size. In these cases, the tentative sizes are undefined, and
can therefore sometimes be zero. However, when resolving the size
constraints, we are already guaranteed to have an intrinsic aspect
ratio, so let's use that instead to calculate the resolved sizes.
