This class uses the concept of shifting to push boxes, which otherwise do not fit on the current page, over the
page-boundary of the next page.
We have two shift positions. The normal shift denotes artificial paddings, inserted into the flow where needed to
move content to the next page. The header-shift is inserted when a repeatable table-header is processed. This header
reserves a virtual padding area in the infinite-canvas flow to push the next assumed pagebreak to the y2-position of
the header. A header-shift modifies the pin-position on a box, and modifies where pagebreaks are detected.