scim setup (Gnome/GTK+)

Run scim-setup, open Panel > GTK, and section ToolBar:
There's an option of always show. But not an always hide.
For someone, just systray will be enough and the toolbar is nothing more than extra disturb. You can turn them all the buttons off from toolbar, and make it auto snap to screen border. You now may ignore the one button of toolbar at the corner.