Perhaps you need to read the code. One can't tell FLTK to add width to this edge or that edge, you can only tell it the new size is XxY. Further, the -g code is completely outside the apps themselves, in FLTK.
The previous reference to specifying an edge to adjust is an indication of my unfamiliarity with FLTK. Such references are probably now best disregarded. As a non-coder reading the code is not open to me. I offered the suggestion based on observed behaviour in which the app makes the choice.
The screen shot provided in reply #2 was the result of 200x100-0+0 i.e smaller than the default of flnotify. It produced a box of the correct size but not the desired location. Interestingly, the top-left corner of the box is located in the same position as a box of the default size. Is this a coincidence?
It is almost as if a box of of the default size is first created with its top-right corner referenced to -0+0, which in turn produces a location for the top-left corner. Secondly a box of the user specified size is created, from a reference point of the top-left corner of the default box. Because this is shorter, the location of the top-left corner is unaffected, resulting in the gap between the top-right corner and the edge of the screen.
The attached screen shot uses 500x300-0+0 i.e. larger than the default of flnotify. It produces a box of the correct size and is in the desired location. Here the top-left corner is not at the same location as a default box.
It is almost as if a box of of the default size is first created with its top-right corner referenced to -0+0, which in turn produces a location for the top-left corner. Secondly a box of the user specified size is created, from a reference point of the top-left corner of the default box. Because this is longer, the location of the top-left corner must change to accommodate it in order to remain within the screen boundary, resulting in the box presenting as specified.
If the conclusions drawn from the observations are correct, reducing the size of the flnotify default box size will (largely) overcome the matter as the user will specify only sizes that are larger than the default.