I tend to use Plasma Desktop in its default configuration since that is what most people are subjected to. However, when I first started contributing to kicker (kde2/kde3's panels) I started doing so because I used autohiding panels on multiple screens and .. there was room for improvement. Since autohide panels are not the default, I no longer used the feature on a daily basis; basically only when bugs were reported would I turn it on.
Well, I decided to spoil myself the other week and turn on autohide on my one lonely panel. Besides having that screen real-estate back for the first time in many years (ok, on a 19" monitor it isn't that much real estate. ..) I got to enjoy all the improvements that had accumulated.
There is the nice blue glow that appears and intensifies as you move the mouse closer. These days this is a kwin effect and boy is it nice and smooth. It's the kind of feedback I could only have dreamed of with kicker.
The next thing I noticed was the hiding animation itself. With kicker we just moved the panel around visually on-screen. This was very funny when you had two screens next to each other as you could watch it move from one screen ... to the next! See, it wasn't really hiding. We fixed that with Plasma Desktop: it actually really hides now. We went one step further and delegated the animation to the compositor. This allows it to be silky smooth and take as little CPU as possible. Ah, performance ...
With kicker, autohide panels would wake up the system a number times per second. This was because it polled for the mouse position to know when the mouse hit a screen edge. Waking up a process means waking up the CPU, which is not good for your battery, and it means keeping it always resident in memory which impacts the performance of multitasking. It's much better when process remain silent, and so with Plasma Desktop we use input-only windows and XEvents to trigger the unhides. Beauty.
Well .. sorta. These input-only windows can interfere with other windows in specific situations, though we've minimized those cases over time. Still, we're going to make this better .. and get rid of the X11 dependency all at once (which is important for supporting Wayland) by moving screen edge handling to the compositor, which in our case is, at least by default, KWin. This means that the compositor will be able to coordinate the desktop shell's screen edge needs with the window managers with .. well .. anything that needs screen edge handling. All without fusing the shell and compositor.
One more gripe I had with kicker is that if you were especially evil (or just unlucky) you could trigger subtle bugs with the hide/unhide mechanism. I worked a lot of annoyances and bugs out of that code, but it was never quite 100% perfect in 100% of cases. So far, I've been unable to trip up Plasma Desktop's panels. Yay.
So autohide panels may not be the biggest feature or the most required feature, but it's been a nice little frill for me these last couple of weeks. :)
Update: I totally forgot to mention what is probably my favourite feature of all: now whenever there is a notification or something requires your attention, the panel containing it automatically unhides so you notice and then re-hides itself ... unless you aren't interacting with the system. In that case it stays unhidden until you move the mouse or type something (or generate some form of input) .. so you never lose a notification just because you were away from your desk. MAGIC!