I'm under KDE Neon 6 and it seems that the Enter next to the numeric pad is not working.
Spine is the only program in which I have this issue and the same key works fine in the Windows version.
Spine seems to be detecting the key though, since I spot the cursor restarting its flashing loop.
The Hotkey Popup doesn't return anything when I press it, while it returns Search: Enter when I use the keyboard Enter.
Are you setting up your own hotkeys? If so, ENTER and NUMPAD_ENTER are separate keys:
http://esotericsoftware.com/spine-key-names
You can map the same hotkey to both by listing the hotkey twice. We'll add those entries in 4.2.21.
Expand/Collapse: ctrl + shift + ENTER
Expand/Collapse: ctrl + shift + NUMPAD_ENTER
Search: ENTER
Search: NUMPAD_ENTER
Search - Select All: ctrl + ENTER
Search - Select All: ctrl + NUMPAD_ENTERIf you are wanting NUMPAD_ENTER to work like ENTER for built-in Spine functionality that you don't have control over, let us know and we'll see about making NUMPAD_ENTER also work there.
No, I'm just used to use the NUMPAD_ENTER to confirm values I input in text fields, usually in the transforms or when there are many numbers to type in names.
So I guess the function I'm missing in this case is whatever function ENTER does that makes Spine apply the value and show me the result.
I always set up custom hotkeys as first thing when installing Spine, but those are for other functions, not involving the ENTER key.
I guess it's not a big deal and one could get used to press the regular ENTER after having used the numpad, but in the Windows version it worked out of the box so I was wondering why it's not the same here.
By the way I tried to assign the Search function to NUMPAD_ENTER as well and it works perfectly, but still doesn't confirm values in fields.
Gotcha, we'll get that fixed, thanks!
Personally I use a keyboard without a numpad, so I've never noticed. 
Hello again!
I've noticed the NUMPAD_ENTER now works when confirming values in the transform fields, thanks for that!
Would it be possible to fix it for all the other cases as well?
Entering values in the Properties section of the Tree, in the Weight value field, and I don't know which other case I'm missing. But basically I think it would make sense to have that button behave exactly like an ENTER everywhere 