I was just offsetting a line and this yellow box appeared around one of the points. What does it mean? Everything seems to be working as expected.
In this particular sketch, it could mean that there is a zero line length segment. ie, Is it possible when you made the vertical line, you started with 3 segments instead of the two that are shown now? If so, then drag select (from upper left to lower right) in that area and delete the segment.
Hope that helps.
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