More future plans.
Gave Listboxes a border on Mac OS X. That just leaves GTK+: oyxgen-gtk always gives GtkScrollingWindows a border, which misled me earlier... other themes don't.
Noted that NSScrollView sets the Listbox border in listbox_darwin.go.
Removed the bit about selectable messagebox text from the TODO file: that's a platform-specific thing.
More TODOs.