If one inserts a sufficiently long set of Tags in the tag editor (total number of characters being important rather than the number of tags), then eventually one of the tags will wrap down and move to the second line. One can tell that the tag will wrap because the current tag one is entering will stop being given in full, and will start scrolling in the area from the end of the previous tag.
Once one has a tag at the beginning of the second line, the number of characters that one can enter in the tag after that before this same kind of mini-scrolling occurs, is not determined by the available space to the end of the second line: instead the limit is at most the number of characters that were in the tag that wrapped down (and appears at the beginning of the line). For example if that tag happened to be 9 characters long, then the mini-scrolling would start after the 9th character of the tag one typed in afterwards.
In most of my tests, the limit was at most 1 character less than the previous tag, and occasionally even fewer were permitted; I only observed the maximum length once.
The field-scrolling at the end of the first line turns out to be a problem in itself. If the previously entered tags happen to take up most of the first line, then one can end up with just one or two characters available to enter the new tag in, with the entered tag being impossible to read during entry due to the scrolling in the narrow field.