Extended monitors on Macbook pro

Although it is not mentioned in detail in that article, you can have BOTH Extended and Mirrored at the same time if you have three or more displays total. Use the Arrangement pane.

"Normal" Extended displays share an edge.

"Mirrored" displays look like a double image of one display. The alternate way to mirror two displays is to drag the tiny blue box representing one display until it nearly coincides with another. Those two should mirror. This may cause the resolution on one or the other (or both) display to switch to a compromise resolution.

nate4re wrote:

What could I be doing wrong?

What are you doing?

This is how to connect an external monitor. To mirror a display you click mirror display. To use extended desktop you unclick the mirror display.

Use external monitors with your Mac - Apple Support

Did you try it? does it work as intended? is there any more to the story that is not mentioned?

Great, thanks for that.

Most of us don't have enough displays to test whether that feature really works as they say it should.

When you say expand the window to "Full Screen" do you mean using the "Full Screen" feature? That feature has a really weird History to it.

Did you try just stretching the window out to near the full size?