Might be useful to us to have more context for what you are actually doing; do you have some small, compile-able code samples, for example, that would illustrate what you are doing, and perhaps some descriptions of what effect you want to attain...

In general, if no widget in a container (i.e. a group or window, usually) is identified as the resizable, then any resizing it shared out amongst all the widgets.

The "usual solutions" are to either make the window non-resizable, or to assign a widget to "consume" the resizing (sometimes a hidden widget in the group is used to achieve this.)

There's a How-To that might help a bit : https://www.fltk.org/articles.php?L415