on-focus in drscheme:unit:interactions-canvas%
Called when a window receives or loses the keyboard focus. If the
argument is #t, the keyboard focus was received, otherwise it
was lost.
Note that under X, keyboard focus can move to the menu bar when the user is selecting a menu item.
( -> void
send a-drscheme:unit:interactions-canvas on-focus on?)
on? : boolean
When the focus is on, calls
make-searchable with this.