Mapped sources/targets don't show connecting lines when one of the elements is offscreen

[jpav]

If there is a mapping between a source and target, but one of the two elements offscreen such that the user must scroll to see it, then no connecting line is shown currently, leaving no real visual indicator that a mapping even exists between those two elements. If a source element is mapped to more than one target element, and the targets are far enough part that they cannot be shown on the screen at the same time without scrolling, then scrolling will actually cause the connecting line to bounce between each target element as it it shown or disappear completely when neither is shown. One solution to this problem might be to always show connecting lines when a mapped source/target element is highlighted, cutting off the line at the edge of the screen where it would be if the area were unbounded and scrolled just like the source/target trees.