copy-to-right is including a prior non-selected diff in the copy

[michael-crawford]

I've been using split-diff's copy-to-right feature to modify a bunch of files over the last few days, and this has uncovered yet another bug. This one is best explained by the PNG I'm attaching, so please see that image to follow along.

1. In the left pane, on Line 156, I have a line containing "getopts" - options to parse in a bash script. This initially differed from what's in the right by both the "m" and "Z:" options. I'm adding the m option, and the Z: option difference must remain, so I manually edit line 158 in the left pane to add the m option in the correct location. When done as shown in the attached image, the "Z:" difference remains.
2. In the left pane, I click on line 162, containing the case which handles the m option, then click on the copy-to-right button. This should ONLY copy line 162 in the left pane to a new line between lines 163 and 164 in the right pane.
3. However, what it does, is not only what I expect, but ALSO it copies line 156 containing the getopts line in the right pane to line 158 containing the getopts statement in the left pane - the prior diff is also included in the move.

This behavior happens frequently, but not always. More often than not, and just in this one situation. But I think it's been happening a lot, as I keep finding option flags which are missing, and I think it was caused by this happening on prior editing passes when I didn't notice and was not double-checking. Now that I am, I'm seeing this happen quite often, so I thought I'd report it.