@andy5995: Yes it does.
Tested with your repository branch ```debugger-gtk3```: - Ubuntu 18.04.02, gtk 3.22.30, glib 2.56.3: the arrow is missing - Ubuntu 18.04.02, gtk 2.24.32, glib 2.56.3: the arrow is showing
Just in case you did not notice because "the down arrow is missing" does not describe the problem I noticed very good. I would say enabling the ```debugger``` plugin on gtk3 causes the menu bar in the message window to have a fixed size. It is not shrinkable/growable any more. So the arrows are gone. Also the menu items are not reduced any more if the height of the message window is too small (I mean that usually e.g. only two menu items are shown if there is only space for two and then you can use the arrows to reach the other buttons). Disabling the ```debugger``` plugin immediately brings back the usual behavior.