@Kovid - its more about the colors than the underlining styles - that was just another example/reason. I use numerous colors in my highlighting for different types of highlights (after all, why else would multiple, customizable, colors exist?). So currently I have to make twice the clicks (while having to navigate to the colors in the 2nd window) in order to do any highlighting.
Most of your objections are quite reasonable - say, single vs double clicking on a highlight - but this one just doesn't make sense. It is standard for any application, be it an ereader or word processor, to have the primary formatting (frequently used) options on the main toolbar - you don't click an extra menu to access bold, etc... We've shared screenshots of at least 5 popular ereaders that have the colors on the popup bar - this isn't arbitrary design, but is instead rooted in user functionality.
Again, thanks to your very reasonable compromise of allowing the user to customize their own popup bar (after listening to our feedback for creating the wonderful popup bar to begin with), any complication of the bar is up to the user - if they want 1 or 10 highlighting options, that's their choice.
If, on the other hand, it is a large complication on your end, that's another story. You're, of course, not obligated to implement this or anything at all, and we're all very grateful for all of the hard work you've put into this feature and Calibre in general. But you've asked for feedback and we're telling you what would be helpful. I'm thrilled with not just finally having this highlighting/annotations feature, but the tremendous evolution/refinement of it. This would be the final step in that process and I hope you'll consider implementing it (even if just to get me out of your hair...
)
Thanks again for the great work!