GtkMenu layout broken in 3.24.10

Something seems to have broken between gtk 3.24.9 and 2.24.10.

Renderings from the "Menus" gtk3-demo on a Windows 10 machine with packages from MSYS2's mingw-w64:

• mingw-w64-x86_64-gtk3-3.24.9-4
  
• mingw-w64-x86_64-gtk3-3.24.10-1