(3) I agree with Manolo that we should investigate and understand why such old code triggers a bug (crash) in your code. Understanding what happens is important to know *what* exactly needs to be fixed. Maybe there's another bug elsewhere (in FLTK or inyour application) that causes this unexpected behavior. As Manolo wrote, it is possible that some code accesses a deleted widget or window which causes undefined behavior. Do you have any insights what may trigger the bug in your program?

The problem seems to be that the Window is hidden while being in an FL_LEAVE event of another window.

OK, let's recap and try to simplify the scenario, maybe to create a small example program that triggers the issue so we are sure and can test easier:

(1) You have a (main) window A (subclassed from an FLTK window class)
(2) You open another window B (the small preview window)
(3) Window B is a modal (or non-modal) window (stays on top of A)
(4) You drag some "object" (in the timeline) inside your window A
(5) Your window A gets an FL_LEAVE event when the mouse cursor leaves window A while dragging
(6) You hide window B while handling FL_LEAVE in window A's handle method

Is all of this true? If not, please correct my wrong assumptions.

I believe so.  Here's a self contained example showing the crash on MSVC and compiled with MSys64 as my terminal as usual:

https://mega.nz/file/OaJ0XKwa#qgxJK2q9E9fzqO3Ltf5wUA2HR4ts4cCHWLc9lvoRIU4

More questions: which of the windows (A, B, or both) are OpenGL windows?

The timeout was a work-around (which randomly failed too) to hide the window.

According to your video with the crash you removed the timeout and did it as described above. Is this correct?

It would be great if you could provide us with some more information about the program's state when the crash is triggered. Thanks in advance.

I think indeed it was a Windows issue (bug).  Sadly when I wrote the timeout code, I did not document *why* I wrote it and had not delved into FLTK's code.

Now that you can reproduce the issue, can you also reproduce it on Linux (X11 or Wayland) or macOS?

Note: I'm asking these questions because I wonder why this bug is triggered in your code and was not reported earlier. Meanwhile I'm pretty sure that the fix given above is correct and necessary but I'm still wondering what the root cause of the issue may be.

-- 
Gonzalo Garramuño
ggarra13@gmail.com