Gonzalo: I believe we must know exactly the scenario that produces the crash,
even more because it happens in 22 years old code. The code in question
searches "what really turned invisible". This means that it’s expected to run
only for a window below something in a widget tree that has been turned
invisible before. In that situation p is never NULL because p->visible() is
false before p gets replaced by p->parent(). So what exactly is the scenario
in which this code is run by this particular client app? Isn’t there something wrong before?

Manolo

The problem happens when calling hide on a window from within an FL_LEAVE event. I was working around the issue with a timeout but it would still sometimes fail.

Here's a video of the issue (with the console printing out the Fl_Window.cxx nullptr):

https://mega.nz/file/7DhlCLiS#yWd1k-n10M86QQBO0gbjx-mDZNk99P-gP4ZnZhWApxs

As you move or enter the timeline a thumbnail appears, like on Netflix. The problem is calling hide when exiting the the timeline (ie. FL_LEAVE of the timeline widget). Note I never destroy the thumbnail, just hide it.

-- 
Gonzalo Garramuño
ggarra13@gmail.com