Within each toplevel window, one widget at a time may
have the keyboard focus. Any
key events received by the toplevel window are
forwarded to the focused widget. This is important
because typing something on the keyboard should have
only one effect---changing only one text entry field,
for example.
Most widgets will give some visual indication that they
have the current focus. Using the default GTK+ theme,
the focused widget is typically surrounded by a thin
black frame. The user can move the focus between
widgets, using the arrow keys or the Tab key. Focus can
also move to a widget if the user clicks it.
The concept of focus is important for keyboard
navigation. For example, pressing Enter or the space
bar "activates" many widgets if they have the focus;
you can move between buttons with the Tab key, and
press one with Space, for example.
