Thanks for your additional comments, folks.
@WillyTchiYuanHa, with Balsamiq 3, we made the decision to position the UI Library at the top. We actually had it on the side for a while to preserve the vertical scrolling, but from a lot of feedback in our beta, decided to put it at the top. Here’s our announcement of the change: New Feature: "Panels" UI - #33 by peldi. You can look farther up in that thread and see the discussion that led to that.
@bencegh, we realize that it is challenging. We do have a feature request on our list for the future to support this better. Because we are based on Adobe Air, this is quite a bit more difficult technically. We are starting work on creating native versions of our app, and this will be much easier, and work better in those versions. They’re a long way off though—it’s a huge project for our small team. We’ll continue to see what we could improve in the meantime.
For now, I would suggest giving Quick Add a try. We added a small thumbnail preview to the list, so as you search for something, it’s much easier to identify what you want. Just use / or + to put focus in the Quick Add search bar and start typing. You can even press SHIFT+ENTER to have the control placed on the canvas where your mouse pointer is. Hope that helps!