I think most of us can't read rust, but offhand it looks like you're creating the buttons in the table's draw code which I wouldn't recommend: the table's draw function only triggers for visible items, and are called repeatedly during redraws.

Please look at the example program examples/table-as-container which creates button and input widgets that extend beyond the window borders, where scrollbars appear and work normally during scrolling and column/row resizing.

Note that you may not actually NEED an Fl_Table widget if you're just making one widget per 'cell' in the table. You could just use an Fl_Scroll and create the table of buttons in the needed positions and that should work fine. You might only need an Fl_Table if you need e.g. resizable rows/columns or something.