[Albrecht:] Here's a new idea (untested): we could first send an FL_SCROLL_GESTURE event and if this is not handled by any widget, send an FL_MOUSEWHEEL event. This way "new" programs can handle the FL_SCROLL_GESTURE event and "old" programs can still handle FL_MOUSEWHEEL events. It's similar to FL_KEYBOARD events that are converted to FL_SHORTCUT (if not handled) and finally to FL_SHORTCUT with flipped case. Does that sound sensible?

I hope it will. We have the precedent of FL_KEYBOARD and FL_SHORTCUT, so I believe it should (a) be doable and (b) work for users. It's a little overhead to send an event that nobody uses (if both FL_SCROLL_GESTURE *and* FL_MOUSEWHEEL are not used). But that's a small price.