Applescript - the activate command makes application "half-active"
Solution 1:
Move the activate command outside the tell application "System Events" block:
tell application "System Events" to tell (process 1 where frontmost is true)
click (button 1 where subrole is "AXZoomButton") of window 1
end tell
activate application (path to frontmost application as text)
Or you could probably just remove the activate command completely.