As topic suggests.
That particular button does nothing exciting.

Well... not entirely true.
It seems to "move window to same monitor and take the window out of maximize."
If I have an maximized application and press this button, the window blinks, stays the same size on the same monitor, but is no longer maximized.

W7 enterprise x64
DF 3.1.7.106

These work as they should.
"move window to previous monitor"
"move window to next monitor"
"move window to next monitor and size proportionally"

edit: added screenshots