I've recently updated the translation of Frescobaldi and I found out a wonderful feature that I had ignored for a lot of time (I don't know when it was introduced..).
I'm talking about Window>Split horizontally, which allows to divide the input view in two scrollable panes. This is _very_ handy when writing a polyphonic music where each voice is saved in a variable; after a few bars the distance between the two voices forces you to scroll up and down between the first and the second voice, while inserting new bars. If you split the input view, you can easily jump from first to second voice without scrolling.
I'm wondering how many other key features provided by an editor I'm missing..