On my laptop, I use KDE 4 on Linux Mint 17.3. I have an Nvidia NVS 5400M, along with the integrated Intel HD graphics. I have the latest Nvidia binary driver installed, 367.57, and I've got the Nvidia GPU selected. I do not use bumblebee.
I use two external monitors at work and at home, one connected via VGA, and one connected via mDP adapted to DVI. If I switch to Intel graphics, the mDP output is no longer functional.
With this setup, multiple monitor support is somewhat flaky. When I dock or plug in the external monitors, I usually have to fiddle a bit in order to get everything working properly. I'm used to this, I know it's because of bugs in Intel's driver along with bugs in KDE (and maybe in X, too). I'll wait patiently for these to get fixed.
But at work, I recently got two new monitors. The weird thing that's happening now (and it's only happening at work), is that my desktop spans all three monitors. My panel stretches across all three, as does my wallpaper, and when I maximize or full-screen windows, they do, too. I have a huge, 5760 x 1080 desktop.
When I search, I see old forum posts from people asking for this feature, and other people telling them that it's not supported. Well, it looks like it must be supported now, but how do I turn it off?