Sorry for being so late to the game after Albrecht already released the code, particularly because I checked the code much earlier, but in retrospect wasn't focussed enough. So here are my suggestions for an API, but this will probably break existing code.

So, Albrecht, if you want to keep the API, I am ok with that too.

We have `margin(int, int, int int)` and `margins(int*, int*, int*, int*)`, one to set and one to retrieve all margins, and `margin()` to get the left margin, and `margin(int, int=0)` to set all margins equally and optionally the gap. Shouldn't we either use the singular `margin` for all cases, or use `margins(int, int, int int)` instead of `margin(int, int, int int)`?

We have `set_size(Widget*)`, `set_size(Widget*,int)`, and `set_size(Widget&, int)`. `int` has a double meaning here. If it's 0, don't use a fixed size, else be fixed. My suggestion is `int fixed(Widget*) const`, `void fixed(Widget*, int yes)`, and `void fixed_size(Widget*, int size)`. This takes away the special meaning of size 0. The naming seem more explanatory to me, and it follows the FLTK rule that getters and setters have no get and set prefix.

The first form returns 0 or 1 if a widget is flexible or fixed.

The second form sets a widget fixed to its current size. This is neat because the constructer requires a width and height anyway, so just use that.

The third form can set a fixed size of 0 as well, because `fixed(w, 0)` can make a widget flexible again.

And finally, I would like a `int size(Widget*)` that returns `w()` or `h()`, depending on `Fl_Flex::horizontal()`.

Thanks for reading.