I have a Prompt for User Input with a couple of popup menu items (action:Prompt for User Input [Keyboard Maestro Wiki]). For the item which contains a string of numbers (separated by the |), I cannot use the down or up arrow keys to scroll up and down the list. With the mouse I can. For example, pressing the down arrow does not nothing. However if the items are text items (separated by the |) then it is possible to use the down and up arrows to go through the list.
It prompted me to do some testing. I could not get the CTRL key approach to work consistently.
But I did discover this:
###When the focus is on the popup menu, use the UP ARROW ▲ key to OPEN the popup, then you can use the ▲ or ▼ keys to select the item.
Pressing the ▲ key shows the popup menu:
EDIT: 2016-04-24 17:15 CT
That worked very consistently about 95% of the time for me on Yosemite 10.10.5.
There is still some strange behavior/conditions that sometimes prohibit the arrow keys from working. I ran many tests, but can not figure it out.
Everyone should keep in mind that you also need to enable "TAB to Controls" in the System Preferences > Keyboard > Shortcuts:
the arrow up solution works for me (macOS High Sierra). It would be more coherent with my typing behaviour though if arrow down would work as well. Maybe @peternlewis could look into this for a future release.
The behaviour of this is all handled by the system. For some reason it has never behaved well in Keyboard Maestro Engine’s windows, but it is not something I can fix unfortunately (well, maybe if I knew what it is that makes the system behave poorly then I could find some way to not tickle whatever system bug is causing the poor behaviour, but since I have not the slightest idea what it is about Keyboard Maestro Engine that induces the poor behaviour by the system, I can't see there being any solution).
Choirs of angels herald your 2016 wisdom, @JMichaelTX! Hadn't even occurred to me to check this System setting until I stumbled upon this "offhand" comment! Problem solved!