FYI I built and ran your program and see the same thing. My *guess* about the FLTK internals here is that changing font size does not cause a resize of the widget *nor* its child components -- in this case, the internal area that is scrolled. If so, then possibly a clumsy workaround ("kludge"?) for your application when changing font size, is to create and add a new instance of the browser with same X,Y,W,H and with new font size, then copy in the text lines from the old browser, the scroll to the old line, then show the new browser, then hide / delete the old browser, then call the parent container group's init_sizes() method. Occasionally I have to employ tricks like this when reconfiguring things in a UI.  Just part of programming life. But normally, I only manipulate font size in the other direction, ie. in *response* to resizes, rather than resizing in response to font size changes.