[QGIS-Developer] is_layer_active expression

[Otto Dassau]

Dear Devs,

in QGIS 3 I found the expression "is_layer_active" within the copyright
decoration very useful. I would like to use this expression also within the
print composer, but it seems to be not available there.

Is this correct, are expressions not available all over QGIS within the
expression builder? And how can I get this expression working for the print
composer as well? 

Thanks for any hint.

Regards, Otto