Relevant to: DOSBox v 0.74, 0.74-2 on macOS

For games that use Ctrl and arrow keys (such as Commander Keen), I noticed that unless I pressed the left/right arrow keys before pressing the Ctrl key, only the Ctrl key press would register and the left or right arrow key press would be ignored. I replicated this across multiple Macs with different macOS versions.

The apparent cause is an OS-level keyboard shortcut (Ctrl+left/right arrow) introduced in OS X Lion to cycle through desktop spaces. If you routinely have multiple desktop spaces or full screen apps open while using DOSBox then you probably noticed this connection right away - I don't so I didn't.

To fix this you have (at least) two options:

Option 1: Disable the system desktop cycling keyboard shortcuts by going to System Preferences -> Keyboard, then go to the Shortcuts tab and navigate to the Mission Control section. Uncheck the "Move left a space" and "Move right a space" shortcuts, or edit the shortcuts to a different key combination.

Option 2: Open KeyMapper in DOSBox using the keyboard shortcut Cmd+Ctrl+F1 (or just Ctrl+F1). Click on the Ctrl button in the GUI, then click the Add button and press Z to remap the Ctrl event to a Z keypress (or any other unused key of your choosing). You might want to also remap the Alt key to X to preserve the spatial relationship.

Have fun out there.