The setup of a Cintiq Pro with an older iMac can be tricky. This tip describes how to manage occurring problems.

Connecting the Hardware

I own a rather old iMac without USB-C. Thank to the Wacom Link Plus Adapter (included in the Cintiq Pro 13) the connection was quite simple. Just make sure to have a free regular USB slot as well as a Mini DisplayPort.

Setting up the displays (refers to macOS High Sierra)

A Cintiq Pro is a second monitor. Do not confuse it with an additional desktop (used by Mission Control)! The cool thing about having more than one monitor is, you can arrange the displays as you like. Go to the System Preferences, then select “Displays” → “Arrangement”.

Mirror Display

The easiest option is to mirror the display which means you’ll see exactly the same on your iMac and your Cintiq Pro. Click on “Mirror Displays” and your done. However, this method has its disadvantages. First consider that the resolutions of the displays may not match. Secondly you don’t have the advantage of a second monitor. I prefer the second method described below.

Arranged Displays

Two arranged displays are actually two different desktops. That can cause some troubles. I wanted the desktops to be arranged like this: On the top the iMac, on the bottom the Cintiq Pro - corresponding to the arrangement on my workspace.
Note you can arrange the display as you please, top - bottom or left - right. Also note you can change the main menu bar (represented by the white bar). See below why you should care about that.
Once arranged correctly, you can move the mouse below the bottom border of your iMac display and it should appear on the Cintiq display. The same goes with the windows. Simply drag them to the desired display.

The Main Menu Bar

When I started Clip Studio Paint I couldn’t move it to the Cintiq Pro display because you cannot move its command bar (the bar on the top showing common icons). Every other window can be move, but not the command bar. Frustrating. I googled, but none of the tricks worked (such as "force a program to start on the second display").
Eventually I dragged the main menu bar in the display settings to the second display (in my case the Cintiq Pro, see above). Now every program starts on the CIntiq Pro display and I can use Clip Studio Paint as expected. Other programs such as Finder or Firefox also starts on the Cintiq Pro display. I simply drag them to the iMac display.

Unexpected behavior

Unfortunately not every program runs correctly on a second display (which is now the iMac display). Expect strange behaviour! For instance my music player refuses to show the context menu. It only works correctly on the Cintiq Pro display.
Also keep in mind that the Dock (in my case at the right position on screen, automatically hide and show enabled) is only shown on the main display (the Cintiq Pro display). The same goes for system dialogs such as the file copy dialog.

Turning off the Cintiq Pro

What happens to the windows on the Cintiq Pro display if you turn it off? In my case usually nothing happens at all. All windows on the Cintiq Pro display are simply gone. I consider this to be a bug. There is simply tick to bring all your window back to the iMac display, though.
1) Turn off the Cintiq Pro.
2) If you miss windows as well as the Dock, press Shift-Ctrl-Eject. That turns off the display.
3) Press Space to turn the display on again. All your windows should be shown again, because the iMac display is now your primary (and only) display.
The same can be done by unplugging the Cintiq Pro.

Enlarge the Mouse Cursor

A Cintiq Pro 13 has a quite small working space. Since the display resolution is high in contrast, icons, buttons, fonts and other user interface elements may be very small. I haven’t found a way to enlarge them. However, what can be enlarged is the mouse cursor. Go to the System Preferences → Accessibility → Display and adjust the cursor size.

The Preview Trick

When I work with Clip Studio Paint on the Cintiq Pro I open the exported JPG image in Preview on the iMac display to see how it looks on a regular monitor. You can simply click on the Preview window and it will reload the latest exported image automatically.