GC failure in Gtk (1.6)

[timholy]

1.6-specific failure described in JuliaGraphics/Gtk.jl#562, which includes a reproducible test case.

git bisect identifies #38180.

[timholy]

TODO: if #38180 is being kept, JuliaGraphics/Gtk.jl#562 (comment) seems to suggest this is a minor breaking change (for the sake of a bugfix, which is acceptable). But it requires a NEWS entry that summarizes the change and what mitigation strategies need to be pursued in packages. Assigning @vtjnash.

[JeffBezanson]

I believe this is fixed by JuliaGraphics/Gtk.jl#569