To the best of my knowledge, notepad++ isn't really designed to work in place of an IDE, which will manage documents like this a little better for you.
Otherwise, if you're hell-bent on using notepad++, browser through the plugins. Plugins menu => Plugin Manager. Perhaps you'd prefer to uncheck the muli-line box? Or ... just deal with it. If this is a frequent occurrence for you, one would think you'd eventually grow accustomed to the rows reorganizing themselves.
Or just don't keep leave so my files open When I find I've got that many tabs open in a text editor (or even an IDE), it's usually b/c I'm trying to work on too many aspect of a project simultaneously.
... Oh, or get a wider monitor. I can open around 20 tabs before notepad++ adds a row. And I'm only on a 22-inch or something like that.