2 things I noticed:

- The orientation changes (HORIZONTAL -> VERTICAL) when adding widgets into the Fl_Flex:

1- create a function

2- add a window

3- add a flex

4- change orientation to Horizontal

5- add a button to the flex. The flex's orientation changes back to vertical

- I had a spurious crash on windows when adding widgets into the Fl_Flex:

1- create a function

2- add a window

3- add a flex

4- create a tree inside the flex