The available width for a line box is originally determined by the HorizontalConstraints struct. However, it is sometimes desirable to override that width constraint. One potential reason to override that width constraint is to force line breaks at particular inline items. (Currently, there is no way to specify to the LineBuilder to break at certain InlineItems, and the only way to do so is to modify the available space on the line.) This is useful for CSS properties such as text-wrap, which need to make line-breaking decisions that do not follow the current greedy approach of filling up the entire available width.