This is probably a known issue, but on the other hand there aren't a lot of resizable objects that don't just tile the whole graphic right now. In fact, there's only one that I'm aware of, and that's the default updraft graphic. However, in the L2 classic style I'm working on, I didn't really see a lot of reason why the water shouldn't be vertically resizable, so I went ahead and set that up. But it doesn't look right in the editor. Works fine in game.
Editor:
(https://i.imgur.com/yueqyhO.png)
In game:
(https://i.imgur.com/AkuMeHl.png)
Known issue, but one I keep forgetting about when I'm actually working on the editor. This topic will be useful as a reminder.
Have identified the cause as being related to the loading of nine-slicing data, but no more specific details than that yet.
Fixed in commit 6ef6b7a.