For the last several weeks my ⌘ + Tab App Switcher stopped appearing on my main monitor and only showed on the laptop screen. At first I accepted this, exhibiting "too-busy" syndrome, but today, with a pain in my neck from staring on the wrong screen for too long, I decided to dig in.

I found out that:

The ⌘ + Tab bar stays on the screen with the Dock you last interacted with. Even if your mouse only hovered over that Dock.

Apparently, I'd rolled over the Dock on my laptop, or secondary, screen and from that moment on it consistently stayed there. Usually, Apple's decisions are smart, and require only a little digging below the surface to find a good answer.

With this, I am totally lost:

Why tie the ⌘ - Bar to the Dock? Shouldn’t it follow the mouse, keyboard focus, or at least the screen with the active menu bar? Apple's implementation is utterly bizarre to me.

At least now I know that I can set which screen my App-Switcher will appear on by hovering over the dock on the screen I want it to be on.

I might have to file a bug about this.

Source: Forum posts about this behavior from 2013.