With touchpad, non-first menu items don't highlight and submenus don't open until clicked

libgtk-3-0 3.4.0-0ubuntu1, Ubuntu Pangolin

1. Open any GTK menu:
    - a pulldown menu, for example the "File" menu in Nautilus
    - a context menu, for example for a file in Nautilus
    - a radio menu, for example any of the menus in the System Settings "Power" panel.

2. Move through the menu with a touchpad.
    What happens: The first sensitive item highlights as it is hovered over, but the others don't.
    What should happen: Every sensitive item highlights as it is hovered over.

3. Move the pointer over a submenu title.
    What happens: Nothing.
    What should happen: The submenu opens after a short delay.

4. Click the submenu title.
    What happens: The submenu title highlights and the submenu opens.

Exactly the same problem happens whether navigating the menu by moving (button down, button up, move) or by dragging (button down, move, button up). The only difference is that when dragging, the release closes the menu without selecting any item.

This is a sequel to bug 949414, which has been marked fixed.