JS bug #4335: text_buffer scrolling with long lines doesn't work

[Jajcus]

When printing a lot of information (I've set the scrollback to 900 lines), and it has started with a very long line (incoming stanza, more than a screenfull), then I can scrollback to the beginning, but once scrolled back, it's not possible to scroll down again.

So, huge line, lots of lines, scroll up to the long line, scroll down -> fails.