Menus and similar thigs, e.g. the Locator popup appear in totally wrong places. While the first menu from the menu bar looks good, things become weird as soon as you move the mouse pointer to a different entry in the menu bar. I also saw Creator crash or the GUI become completely unusable.
Setting QT_WAYLAND_SHELL_INTEGRATION=xdg-shell-v5 does not work at all, QT_WAYLAND_SHELL_INTEGRATION=wl-shell only makes it worse.
Tried with binary snapshot 2185. The same version of Creator built on Qt 5.10.1 works correctly. The menus in Qt tools like Assistant or Linguist look good as well.