Setting up Extended Screen Display
Open System Preferences
Select Mission Control and make sure the following are unchecked
Group windows by application and Displays have separate Spaces
From System Preferences Click on Display.
This opens 3 panels: one for the imac and the two other for the wall and the second monitor as shown below.
From the imac panel above click on arrangement to the default arrangement, extended arrangement, of the three displays:
Take turns to place the cursor on each of the display clicking and holding. A red rectangle will appear on the screen represented by the display.
You may notice that the iMac display is not mirrored. The wall is divided into two with half of it standing alone and the other half mirrored to the second monitor.
Make sure the imac display is on the left end of the arrangement, followed by the unmirrored part of the wall, then the mirrored wall and second monitor. You can do the arrangement by dragging and dropping displays.
While pushing down on “option” key, (option keyboard) drag and drop the unmirrored wall on to the imac display to form the configuration below (Mirrored and extended).
The arrangement appears to be inline from top to bottom but the fact that one display is larger the two other indicates a mismatched resolution.
On any of the panels, click on Gather Windows to bring the windows together.
Make sure the imac window is showing the display option
We are going to align the resolution on all three windows by clicking on scale button while holding down on the “option” key on the keyboard. Our three windows have two different native resolutions but we are going to aligned the resolution on the smaller of both.
3200 X1800 is selected since it is common to all three windows. See picture below
Test to make sure any display is uniform when image is extended from one monitor to the other. You can do this by dragging a word document across to the second monitor. If the arrangement is inline, this will be reflected on the wall. The arrangement window from the imac panel can be used to adjust the arrangement by lining them up as below.