Cairo doesn't autohide on child window overlapping

[ZaZy]

When the main window of an application overlaps Cairo dock, it will hide as expected. When the main windows opens a child window (like, for example, the one that pops up when you are "Saving as..." a file), Cairo will not auto-hide when the child window overlaps the dock. This is a problem with child windows that fills the screen because it's impossible to click any button at the bottom of the window.