With GIMP 2.7 a new bug appeared which manifests in flickering windows when opening dialogs, e.g. the file open dialog.
Looks like the issue is related to that GIMP hides all of its dialogs when the main window loses focus, which occurs when opening a dialog,
causing the dialog to be hidden. Then the main window gets the focus again and shows the dialog, resulting in an infinite loop causing the flickering.