elaforge created an issue (fltk/fltk#1604)

Description of the Bug Report

I've been tracking down a bug where when
I use scroll (two finger on a mac, but of course the event is
FL_MOUSEWHEEL) and fltk then thinks the scrolled window has focus,
even when the OS disagrees. I'll attach an example program, it should
bring up two windows. The red should make it obvious where fltk
thinks the focus is. If you do some mousewheel scrolling in the
unfocused window, then move the cursor between windows to create some
FL_ENTER events, the red should switch over to the scrolled-in window,
and now, assuming OSX with click-to-focus, the OS and fltk now
disagree about who has focus. I tried this in 1.4.4 but also got
latest dev version from github, and they are the same.

I asked claude about this, and it fingered this bit in Fl.cxx:

  case FL_MOUSEWHEEL:
    fl_xfocus = window; // this should not happen!  But maybe it does:

indeed, commenting it out fixes the problem. I asked about
consequences, and it thinks the code is making the assumption that
only focused windows can receive mousewheel events, which is
definitely not true on the mac! It's also talking about
fl_fix_focus() as a way to reconcile with the OS, but apparently that
is not called in mousewheel code path. But of course it would be
better to not get the state wrong in the first place, than to
reconcile! But perhaps on Windows or X11 it is true that you cannot
get a mousewheel on a non-focused window? At least the "this should
not happen" comment seems to indicate whoever wrote it wasn't thinking
about OSX, or at least thought that mousewheel only goes to focused
window.

To Reproduce

Compile

#include <iostream>
#include <FL/Fl_Window.H>
#include <FL/Fl_Box.H>
#include <FL/Fl.H>

static void
show_widget(Fl_Widget *w)
{
    if (w == nullptr) {
        std::cout << "<null>\n";
    } else {
        std::cout << w->label() << '\n';
    }
}

class Bug : public Fl_Window {
public:
    Bug(int x, int y, int w, int h) : Fl_Window(x, y, w, h), box(0, 0, w, h)
    {
        box.box(FL_FLAT_BOX);
        box.color(FL_WHITE);
    }
    Fl_Box box;
protected:
    int handle(int evt) {
        if (Fl::focus() == this) {
           if (box.color() != FL_RED) {
                box.color(FL_RED);
                redraw();
            }
        } else if (box.color() != FL_WHITE) {
            box.color(FL_WHITE);
            redraw();
        }
        if (evt == FL_MOUSEWHEEL) {
            std::cout << label() << ": WHEEL";
        } else if (evt == FL_ENTER) {
            std::cout << label() << ": ENTER";
        } else if (evt == FL_LEAVE) {
            std::cout << label() << ": LEAVE";
        } else if (evt == FL_FOCUS) {
            std::cout << label() << ": FOCUS";
        } else if (evt == FL_UNFOCUS) {
            std::cout << label() << ": UNFOCUS";
        }
        if (evt != FL_MOVE) {
            std::cout << " focused: ";
            show_widget(Fl::focus());
        };
        return Fl_Window::handle(evt);
    }
};

int main()
{
    Bug win1(200, 200, 200, 200);
    win1.label("b1");
    win1.show();
    Bug win2(400, 200, 200, 200);
    win2.label("b2");
    win2.show();
    Fl::run();
    return 0;
}

FLTK Version

1.4.4 and 1.5 github head (c571c2f)

Operating System / Platform:

MacOS Tahoe 26.6.2 (25G83)

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