Well, that's the duty of the positioner class itself - if you use the RelativePositioner for example, it will attach itself to the Component's parent and watch for changes.
Jules the documentation for Component::Positioner and relative component stuff is a bit abstract ...
The purpose of applyNewBounds() is a mystery. Does it get called by anything, or is it there to be called by me ...
And I'm sure you pointed me at the answer before, but the relative rectangle string syntax is referenced as being on this page: https://www.juce.com/api/classRelativeCoordinate.html "The syntax for the coordinate strings is explained in the RelativeCoordinate class." and it ain't ;)
Or is there another bit of the syntax explanation somewhere else, because:
String RelativeCoordinate::toString ( ) const
Returns a string which represents this coordinate.
For details of the string syntax, see the constructor notes.
But there aren't any details in the constructor notes?
And it also references: The string will be parsed by ExpressionParser::parse(). which I think is supposed to be Expression::parse()