Steps to reproduce:
0. log in to a gnome on wayland session
1. open gedit (or evince or any other native Gtk+ 3 on wayland application)
2. press Ctrl+O or the button to open your file chooser

What happens:
File chooser displays, at first with a normal size but topleft on screen. Then the file chooser is resized (way too small, 733x470 px according to Gtk inspector). It can be resized to become bigger, but when it loses focus it is resized to the previous (too small) size again. When gaining focus again, it is often (not always) resized to its original (big) size. When not having focus (small) it still renders a preview in gnome-shell's activity overview that is big.

What should happen:
File chooser should never change its size without user request to do so. It should not be displayed in top left corner, especially not if the application is half-maximized to the right edge. This is working correctly in Gnome+X11 although I can't exactly describe how it goes there.

Additional info:
gtk3-3.18.4-1.fc23.x86_64
glib2-2.46.2-1.fc23.x86_64
gedit-3.18.2-1.fc23.x86_64
evince-3.18.2-1.fc23.x86_64

Additional info:
This issue is present with firefox (Gtk3, X backend) inside XWayland too.
This issue is not present on a gnome+X11 session and does not affect Qt4 or Gtk2 applications.