on-change in editor:backup-autosave-mixin
Called whenever any change is made to the editor that affects the way
the editor is drawn or the values reported for the location/size
of some snip in the editor. The
on-change method is called just before the editor calls its administrator's
needs-update method to refresh the editor's display, and it is also called
just before and after printing an editor.
The editor is locked for writing and reflowing during the call to
on-change.
( -> voidsend an-editor:backup-autosave-mixin on-change)
Sets a flag indicating that this editor<%> needs to be autosaved.