How to run programs in a Sandbox

I would go with VirtualBox. Using Wine + QEMU though in theory is a possibility, I think you'll find VirtualBox will give you:

• better performance
• be easier to setup and use
• better security
• etc.

Perhaps you should look into using the snapshot pausing features of VirtualBox. It's rare that I ever have to start a VM from scratch, rather I resume them, this is much faster then having to do a full on boot.

First note that foreground-window and window-with-keyboard-focus are not the same thing. They are for Mircosoft's Windows, and for a lot of window managers, but not necessarily. For example KDE can manage these two properties separately.

Now about your question, like the point I made above, it will depend on the window manager. In Kde there are setting that allow you to control keyboard focus, even settings that depend on the application (but still part of the window manager). The above image shows how to do the opposite (set accept focus to yes to do what you are asking for). I tested this configured for one application.

Do not mix up the role of window-manager, launcher etc. Both KDE, Gnome, LXDE and others do all of these. The window manager manages which windows are where, there size, whether they are minimise, which one are behind which, which has keyboard focus. The application can also control this, and so can another application, but ultimately it is the process that is registered as the window manager, that is in charge. It can block other processes from doing these things, and decides where a window is mapped, and whether it gets keyboard focus, when first mapped (or ever). The only visible part of the window manager is the title bar(+close, maximise, minimise etc icons) of all of the other windows.