macOS, is my app active?

[hh]

MacOS 10.15.1

This is fld "IN" (your snippet):

tell application "System Events"
  set activeApp to name of first application process whose frontmost is true
  return activeApp
end tell

on mouseUp
  do fld "IN" as "applescript"
  put the result into fld "OUT"
end mouseUp

yields in fld "OUT":

"Livecode-Community" or
"LiveCode-Indy" or
"LiveCode-Business"