The FileChangedRO event:
Fires:
radio_button_unchecked
Before making the first change to a read-only file.
radio_button_unchecked
When making the first change in a buffer
radio_button_unchecked
When making the first change after 'readonly' was set, just before the change is applied to the text.
Notes:
radio_button_unchecked
This event can be used to checkout the file from a source control system.
radio_button_unchecked
Does not fire when a file is changed by an auto-command.