I am trying to create a widget that can be pointed into a box to select the pixel co-ordinates within. It has a click box next to it to activate the point selection crosshairs, and then the point co-ordinates inside the box will be selected.

Any suggestions ?

    I'd suggest deriving a widget from Fl_Group that implements this pixel coordinate picking
    behavior, handle() to track the mouse movement. If I remember correctly, in response to
    FL_ENTER, return 1 so your widget gets FL_MOVE events, which gives tells you where the
    mouse is whenever it moves. Save the x/y coords from FL_MOVE in your widget and trigger
    a redraw() so that your draw() routine can draw() the crosshairs over the child widget.

    Then your app can create this widget as the parent, and make the widget with the
    visual elements to be picked a child of that. (e.g. an Fl_Box with an image() assigned to it)

    For flexibility, you'd probably want to implement this Fl_Group oriented widget
    so that a callback can be set to get the 'picking' events, so your app knows what to
    do when something is "picked".