Maybe this won't strike a chord with you but I've been looking for the K400 equivalent of ultraportable BlueTooth mice for quite awhile. Something that is good enough, compact and fairly cheap. If you haven't surveyed the landscape recently I think you'd be shocked at how terrible most of the choices are. Nearly every decent wireless mouse comes with a USB receiver and that's fine if you're using it with a desktop or even most laptops but for tablets it's not really a practical option (and if you have a full Win8.1 tablet sometimes you want to do precision clicking on the desktop). I tried a Microsoft Bluetooth Notebook Mouse 5000 last year but the scroll wheel was awful, the ergonomics left a lot to be desired, even though it was compact it was still a little bulky/thick and connectivity/tracking was inconsistent. It wasn't a winner for me, but last week I think I found what I've been looking for...
I wanted something very thin with a good scrollwheel as the primary features, a decent size other than the thickness and obviously solid connectivity were also a big deal. I found the HP Slim Bluetooth Mouse (part number F3J92UT) last week and decided to give it a try. After a couple days of light use I'm pretty sure it will suit my purposes well even though it isn't the greatest mouse ever by any means. It's so generic there aren't any reviews online but I think it deserves one so here it goes...
You can see how the top comes off the mouse and that the switches are exposed as well as that the back is held on with magnets, no plastic tabs to break or wear. The one piece design of the buttons is a little squishy, especially as a couch mouse but they seem to be breaking in a little or at least I'm getting used to it. I do wish it had a thumb button or two but for the light usage I'm sure it will get it's not a deal breaker. Paired with my tablet (Lenovo Miix2 8") without issue and it seems to have a solid connection with none of the jumpiness I had with the MS 5000. The fact that it has a really nice feeling scroll wheel with a good middle click is pretty impressive for the formfactor and price. It's super light too, and less than 1" thick, barely thicker than the tablet when its in it's case. Oh, the edges are a little sharp, I might dull them a little with some sandpaper. Ultimately the simplicity of the entire device is sort of charming even if it makes using it sort of underwhelming.
Not too small
Good scroll wheel/middle click
"Buttons" are a little squishy
No thumb buttons
Edges weirdly sharp
Thanks for reading!