When running devhelp, we can see critical messages:

> Gtk-CRITICAL **: _gtk_widget_get_preferred_size_for_size: assertion 'size >= -1' failed

The problem comes from GtkPaned, in the gtk_paned_get_preferred_size_for_opposite_orientation() function. The for_child2 variable must be >= -1, but it can have a lower value:

> for_child2 = size - for_child1 - handle_size;

It can maybe be fixed simply as:

> for_child2 = MAX (-1, size - for_child1 - handle_size);

Here is the backtrace when running devhelp with G_DEBUG=all:

+ Trace 232348