The sytem is fantastic, but I noticed some strange issue that may cause a lot of problem.
In fact, whenever I switch the game to full screen mode any dialogue that should trigger will not show up, but still will be going ( I hear the voice of the dialogue and can even interact with choiche type dialogues). I think it is only a graphical issue. I am working on GMS2.
Hope that you can solve this problem, I would be really gratefull to you.
P.S. I don't speak english that well, so please forgive me if I wrote something wrong :)
Found out how to fix this, have a general global game object.
At the create event put this code: (This will get the width and height of you camera, here the camera is 0)
var _view_width = camera_get_view_width(view_camera[0]);
var _view_height = camera_get_view_height(view_camera[0]);
display_set_gui_size(_view_width, _view_height);
Then at obj_textbox, go to create event and change the scale variable to something that will fit your size, according to your viewport settings.
You will also need to change the fnt_dialogue size to make it fit as well.