I have been playing a bit with the Plasma beta and I noticed that the spacing off in most place (actually even in 4.x).

Screenshot for illustration: http://i.imgur.com/A1TlPez.png (None of the highlights line up)

My suggestion would be to go back to the old style where the highlight stretches to the edge of the widget e.g. like the menu at the bottom of the page.
It could look like this: http://i.imgur.com/allbjVO.png

I really like this idea. Especially because it fits the model of flat design where you want to really make sure that highlights are well represented and using the space available to it. It also makes it so that the user knows that there are no horizontal end to the selection that is being made. It also removes 2 vertical borders which could add extra visual noise to the selection. Have you tried using this idea but mocked up with the visual design toolkit? Check it out!

Yes, thumbs up for this proposal from me too.